An epidemic of mumps had swept down upon the town of Spenceville and invaded the two famous schools, Spenceville Academy and the Wilmot School. Jo Ann got them and so did her chum Julia Wickham, and they were sent home looking fat necked, and Tommy Bassick arrived two days later equally puffed up. They were all about well now but the two schools remained closed, and Julia Wickham had come for a visit to Jo Ann.

Jo Ann lay on a couch with her knees drawn up and a magazine spread open upon them.

"Wicky," she asked, "what do you mean by a sense of humor?"

"What do I mean by it?" Wicky asked. "It's laughing at things, isn't it? You know -- being able to laugh at things. Why? What about it?''

"Look," said Jo Ann.

The article Jo Ann was reading was about wives -- what the modern young man demanded in the girl he would marry -- and the subject was fascinating to Jo Ann. Jo Ann herself intended never to marry; she thought anything like that was too stupid for any use, and she meant to be a bachelor-girl and a doctor or lawyer or something like that, but she was sure that her chum, Julia, ought to get married as soon as she was old enough, and have a lot of lovely children.

"And I'll be a sort of uncle to them, Wicky," Jo Ann said, "and visit you and bring them presents."

"Well, of course, I want to get married some time," Wicky had admitted, "but it's going to be a good while yet -- ten or fifteen years -- before I do. I've got to finish school, and then go through college, and then be a debutante for a while first. And that'll take a number of years."

"But you ought to be thinking about it all the time," Jo Ann said. "I'm all the time thinking that I am not going to be married. Understand, Wicky?"

So now Wicky came obediently and sat on the couch at Jo Ann's side and looked at the magazine article. The article said that several hundred young men had been asked what they would demand in the girls they married and that they had said that, in the order in which they considered them important, they would demand these qualities, all of which Wicky considered solemnly and seriously as Jo Ann read the vital and important enumeration in the periodical before her:

1. A loving disposition. 2. An open mind. 3. A sense of humor. 4. Education. 5. Fidelity. 6. Good health. 7. Energy. S. Good sportsmanship. 9. An even temper. 10. Economy. 11. Up-to-dateness. 12. Neatness.

"I think you've an awfully good chance, Wicky," said Jo Ann. "You've got the whole twelve of them, unless you haven't got a sense of humor. You've got a loving disposition, and you've got an open mind, haven't you, Wicky?"

"I guess I have," said Wicky. "I guess it's as open as anybody's. Opener, probably."

"Yes," Jo Ann agreed. "And those are the two most important things. And you're getting a good education and you've got energy and all the rest -- I wish I was sure about your sense of humor, Wicky. That's the only point I'm worrying about."

"Well, I laugh a good deal," Wicky said. "I laugh harder than you do, when you get the best of Tommy Bassick. Don't you remember how I laughed when Tommy sent us his radio because he thought it was our 'April Fool' to him and the time he opened your Christmas present at his sister Nell's house?"

"Yes, but I don't believe that's the kind of sense of humor this means," Jo Ann said doubtfully. "I don't believe any man -- any young man, anyway -- would demand that you laugh at him when he is looking foolish. I think it means being able to laugh when the joke is on you."

"All right!" said Wicky "I can do that, too. I can laugh when the joke is on me just as well as anybody."

"I'm not so sure, Wicky," Jo Ann said. "Now, I can laugh when the joke is on me. I can laugh heartily. Or I think I can -- I've never let the joke be on me; not often, anyway. I think, Wicky, we ought to have a test. We mustn't leave it doubtful. If you haven't the right kind of sense of humor we ought to know it, so that we can get busy and correct it. It's important, Wicky. It's very important."

"Well --" said Wicky doubtfully. "Well, if you think so, Jo Ann. How are you going to see whether I have the right kind of sense of humor? How can you find that out?"

"I don't know yet," Jo Ann said. "I've got to think about it, but don't you worry -- I'll think of a way."

"And what do I have to do?" asked Wicky.

"Nothing," said Jo Ann. "Just laugh merrily when you see that the joke is on you. And that," she concluded, "will prove that you really have a sense of humor."

"I'll try," Wicky said, but not very happily. "I'm pretty sure I can laugh, but I don't know about the 'merrily' part."

"The 'merrily' part is what proves that it is sincere," said Jo Ann.

For the next day or so Wicky was unusually nervous and uneasy. Usually she was not nervous at all, but now she was constantly expecting something to happen. Before she got into bed she turned the sheet down to see if Jo Ann had put sand or nutshells in the bed, and before she took a hearty drink of water she sipped it to see if Jo Ann had put salt in the water. Whenever she had a minute alone she practiced laughing merrily, saying "Ha! Ha! Ha!" as merrily as she could, so that she might be ready to laugh merrily when the test moment came, but on the third day both Wicky and Jo Ann forgot the sense of humor test entirely. It happened to be a rainy day and Jo Ann was standing at a window when a taxicab stopped next door.

"Wicky!" she exclaimed. "Look there! Ted Spence is coming to visit Tommy Bassick!"

"Oh, dear!" Wicky mourned. "That means trouble. We've had such a nice time while Tommy had the mumps. Now they'll begin to stir things up. They'll be thinking up things to do to you to get even with you for all the jolts you've given Tommy."

"Yes," said Jo Ann, "but that red-headed Tommy hasn't got the best of me very often yet, and I guess I can handle him if he tries any of his tricks on us, Wicky Wickham."

But Tom Bassick and Ted Spence behaved for a couple of days as if they had reformed. They did nothing to indicate that the old feud between Jo Ann and Tommy still existed. The day after Ted's arrival was pleasant and Tommy Bassick shouted across to the girls to come over and play tennis, and Jo Ann and Wicky went over. They had a good game, Jo Ann and Ted Spence playing against Tommy and Wicky. Jo Ann and Ted won, but the game was close.

"Bully game," Tommy Bassick said, grinning, when they had finished. "Play again tomorrow?"

"Yes," Jo Ann said, but when she was alone with Wicky she frowned. "They're up to something," she said. "It's not natural for that red-head to be so sweet. They've got some sort of scheme planned, Wicky; you'll see, but --"

She stopped short.

"But what?" Wicky asked.

"Never you mind what," Jo Ann said. "We'll see."

The tennis game the next day was as brisk and friendly as the first. Jo Ann, who had the muscles of a boy, won again with Ted Spence, and it was not until the game was over that anything happened that was suspicious.

"We can't play tomorrow," Tommy said then. "Ted and I are going to Mud Lake to fish. Father is going to take us out in the car early in the morning and come for us after dinner at night. We're going to cook meals on the island and have a grand time."

"Are you?" said Jo Ann. "That'll be fun."

It was then that Ted Spence took Tommy's arm and led him a short distance away from the girls. He seemed to be making some sort of suggestion to Tommy, to which Tommy was objecting, but as Ted talked, Tommy seemed to agree. The two boys came back to the girls.

"Ted wants to know why you girls don't come with us," Tommy said. "If you want to, I don't care."

"We'd love to," said Jo Ann promptly. "I love to fish. I haven't been fishing this year. We'll go."

When they had left the boys and were in Jo Ann's yard again, Wicky told Jo Ann she was surprised.

"We're not going, are we, Jo Ann?" she asked. "Why, even I could see that they had planned it all out -- pretending to think of asking us just at that moment."

"Don't be silly, Wicky," Jo Ann said. "Of course, I knew that. I'm not dumb. They'll try some smarty trick, and they've got it all planned out, and that's why they've been so sugary and everything. And we're going tomorrow, Wicky, and we're going to let them play their smart trick."

"But why, Jo Ann?" Wicky asked in amazement.

"For your sense of humor, Wicky," Jo Ann replied. "We've got to see if you have one, and if you haven't one it's time we began developing one in you. It's all for your good, Wicky. I don't know what they're planning to do, but it won't be anything very terrible because they haven't brains enough. It will be some silly kid's trick. It'll be a wonderful chance to see if you can laugh when the joke is on you."

"But the joke will be on you, too," suggested Wicky.

"Well, I have a sense of humor," Jo Ann said. "I'll laugh. I'll laugh merrily. The important thing is to see whether you laugh or not, Wicky."

"Well --" Wicky said doubtfully. "You're most always right, Jo Ann, and I suppose you are right this time."

Before the start the next morning Jo Ann encouraged Wicky again, and warned her.

"Now, I won't let them do anything too unpleasant to you, Wicky," she said "but we mustn't do anything that will keep them from playing their little joke on us. We'll simply sit back and pretend that we're stupid for once, Wicky, and give them their chance. We can afford to when we are making an important test."

Jo Ann had dug worms, and her mother had put up a good lunch, and the boys were to take steak and potatoes and corn to cook for dinner. Mr. Bassick was on time and the four fishers piled into the car and were on their way to Mud Lake.

Mud Lake deserved its name. Its bottom was, for the most part, mud and nothing but mud. To reach it one had to drive four miles into the country and through a hill lane. At the foot of the lane were three rowboats. Jo Ann's family owned one and the Bassicks another. The island in the middle of the lake was not large, but it was rocky and wooded and made an excellent camping place. Mr. Bassick drove his car down the lane to the lake, let his four passengers out there, and drove away.

Jo Ann had brought a key to unlock the padlock that fastened the chain that held the boat to the little dock, and Tommy Bassick had brought his key. The oars were under the dock.

"Beat you girls to the island!" Tommy cried when the fish poles and baskets were in the boats, but his boast was one he could not make good. Jo Ann knew how to handle the oars as well as anyone, and Wicky was better than Ted Spence, and Jo Ann's boat was the first to touch land. She did not, for one time in her life, jeer at Tommy for his defeat, and she let Tommy and Ted carry the two lunch baskets up into the wooded part of the island without stopping them. This was rash, and Wicky said so.

"They'll do something to the lunch," Wicky said. "They'll hide it where we can't find it."

"Let them, Wicky, let them!" Jo Ann said. "If that's the best trick they've been able to think up, you'll be able to laugh merrily. We won't starve. Not," she added, "that I'd think it was much of a joke. I'd be ashamed to do a silly little trick like that."

But when the girls walked to where the small stone fireplace had been built under the trees the lunch baskets stood in plain sight and had not been tampered with. In fact, Tommy Bassick did not seem to be thinking of meals at all.

"Get busy, girls," he cried. "You'll have to take care of the food -- put it somewhere and get started fishing. Ted and I are going to get out on the lake as quick as we can get there. You girls got bait and everything? Hurry up -- we want some fish to fry."

"Don't you worry!" Jo Ann scoffed. "We'll be fishing as soon as you -- and get more fish than you'll ever catch!"

"Bet you don't!" Tommy challenged, and he and Ted ran toward the shore as fast as a streak of lightning.

"Jo Ann!" Wicky exclaimed. "They're going to maroon us here! They'll take our boat and leave us on the island!"

"Now, be calm!" Jo Ann said, grasping Wicky's arm. "Remember your sense of humor. Let them take our boat if they want to -- we'll have a hearty laugh about it. Imagine us left on the island! The two girl Crusoes, Wicky! The two bright girls marooned by two silly boys! We'll laugh at ourselves. And that will be the supreme test!"

"Well, I'll try to laugh," said Wicky doubtfully. "I'll do my level best, Jo Ann. but I don't think it would be so awfully funny. Really, I don't! Not if we had to stay all night on the island and it got cold. Or rained. It does look cloudy off to the east."

"Well, I'll laugh, all night or not all night," said Jo Ann, "and rain or no rain. It certainly would be a joke on us -- walking into the silly trap with our eyes open; meaning to do it!"

But the boys had not taken the girls' boat. Tommy and Ted were rowing toward the best fishing ground, and Jo Ann and Wicky got into their own boat and rowed after them. It was not far and Wicky sat in the stern while Jo Ann handled the oars.

Reaching the best fishing ground, Tommy and Ted anchored their boat. There was little or no current in the lake but a breeze swings a boat around and blows it away from the selected spot, and fishing boats usually have a large stone aboard, with a rope tied around the stone, the other end of the rope fastened to a ring in the bow of the boat. Often there are two of these stone anchors, one for the bow and one for the stern, but Jo Ann's boat had only one. Tommy's boat had two and Tommy let down one while Ted let down the other. Then they picked up their fish poles and began to bait their hooks.

"They've got the best place," Jo Ann said. "They would! We'll go as close as we can without getting too near. How about here, Wicky?"

"I don't know. You know the lake," Wicky said.

"This ought to be good," Jo Ann said, looking over the side of the boat. They were in a place where the pickerel weed was not tall, although it was thick all around them, reaching nearly to the top of the water. "Yes, we ought to get some fish here. I'll anchor here."

She knelt at the bow of the boat, lifting the stone anchor that was about all she could raise.

"Never knew this stone was so heavy," she said, and let it slide into the water. The heavy stone plunged downward, dragging the rope with it, but before the stone reached the mud bottom the rope tautened; it hung a moment suspended from the iron ring in the bow of the boat and then, to Jo Ann's amazement, the iron ring pulled out and followed the rope into the lake.

"Well, that's queer!" Jo Ann said. "That ring pulled right out; now we've got no anchor. Not that it matters much with almost no breeze. We can --" She did not finish saying they could fish as they floated. "Wicky!" she exclaimed. "They fixed this ring so it would pull right out when the weight came on it!"

"Look at them!" Wicky cried. "They're pulling up their anchors."

They were indeed. Not only that but as the girls watched them Tommy took his oars and began rowing toward the island.

"Yah, you smarties!" he jeered. "Stay there and see how you like it! Slicky Wicky and Jo Ann the smart! Don't fall in the water and get all wet! Don't get hungry! Catch some little fishies!"

"Had I better laugh now, Jo Ann?" Wicky asked.

"Laugh? What for?" Jo Ann asked.

"To show I have a sense of humor," Wicky said, and Jo Ann looked at her, but Wicky was entirely serious about the whole matter. She waited for Jo Ann to speak.

"If you laugh now, Julia Wickham," Jo Ann said, "I'll throw you out of the boat! But I don't see what they've got to laugh about. It's nothing to lose an anchor. Unless --" she said suddenly, and she took up her oars and began to row with all her strength. "They did!" she exclaimed as Tommy's jeers doubled.

"Oh! What is it? What did they do?" Wicky asked, close to tears. Jo Ann let her oars rest.

"That imp!" she cried angrily. "They've tied the anchor rope to the bottom of the boat somehow. They've fastened the end of the rope to the keel, and now that big anchor stone is down in the mud and tangled in the weeds --"

Suddenly Wicky began to giggle. At first it was a little giggle, and then it was a bigger one, and then a laugh.

"Are you laughing at me?" demanded Jo Ann, fiercely.

"Oh, no! No!" Wicky gasped. "I'm laughing at myself -- coming out here to see if I have a sense of humor, and getting marooned in the middle of a lake."

"If that's what you call funny --" cried Jo Ann, exasperated.

"But can't you see?" gurgled Wicky. "Can't you see what a couple of sillies we look, stuck out here in this boat?"

"Humph!" said Jo Ann sourly. "If that's your sense of humor, I'm glad I haven't got it," and she looked so offended that Wicky fairly shrieked with laughter. Across the water the boys heard her, and Tommy stopped rowing.

"They don't seem to be very mad about it," Ted said.

"They think they've got some joke on us," said Tommy uneasily.

Jo Ann peered over one side of the boat and then over the other, but she could not see the anchor rope. Ordinarily the two girls would have swum ashore -- but the weeds in Mud Lake made swimming too dangerous to attempt. Jo Ann sat thinking while Wicky wiped her eyes and gradually subdued her giggles. The rain had begun to fall gently.

"Yes, that might do it," Jo Ann said.

"What?" Wicky asked with interest.

"Take your fish pole and take the bobber off the line," Jo Ann said, "and fish for the anchor rope. I'll try on this side."

Jo Ann was the one who first caught the rope. And then, very carefully, she pulled upward until her free hand could grasp the rope. Jo Ann and Wicky worked it loose and got it into the boat. Wicky giggled again.

"They'll be surprised when they come for us and find we're not here," she said.

"They're going to be more surprised than that," said Jo Ann. "Now keep still."

Very gently and noiselessly she rowed the boat toward where she knew the island must be and presently Jo Ann came to where the boys' boat was nosing the beach. She swung slowly around until Wicky could grasp its stern.

"Take hold of it," Jo Ann whispered, and Wicky did, and Jo Ann backed away. The boys must have heard something for they came stumbling down the rocks to the shore just in time to see Jo Ann and Wicky disappearing with their boat.

"Here you! Come back here!" Tommy shouted, but Jo Ann bent over her oars.

"Hey! We surrender!" Ted called.

But Jo Ann rowed until she reached the small dock at the foot of the lane. She pushed the Bassick boat away from the shore to float where it chose, and she sent her own boat after it, and the third boat following that. The breeze would carry the three boats away and they could be recaptured only by daylight, and Mr. Bassick was not due back at Mud Lake until about dark. Tommy Bassick and Ted Spence were marooned on the island until morning.

"There!" said Jo Ann. "We'll have to get a ride home somehow, and we won't tell Mr. Bassick until this evening, but I guess that will teach those two boys one lesson. Come on, Wicky. Let's go!"

But before she went she turned toward the island and made a megaphone of her hands.

"Ha! Ha! Ha!" she shouted so that the echo came back from the hills beyond. "There!" she exclaimed. "That's the kind of sense of humor I've got!" and she laughed.