So they all took sheets of notepaper and wrote letters in orange-juice ink. They wrote rather cheeky letters to people they didn’t like, and squealed with joy when the iron made the writing visible and they each read what the others had written.

“Did you really mean to send old Clear-Orf a letter in invisible ink?” asked Daisy, remembering what Fatty had said. “But what’s the point if he can’t read it?”

“Just the fun of the thing,” said Fatty. “He’ll be so wild to get a letter with no writing on it, and he won’t know how to read it. We shan’t tell him either!”

Fatty wrote out his first letter to Clear-Orf again, sealed up the apparently blank sheet of paper in an envelope and printed Clear-Orf’s name on it.

“It’s rather a silly thing to do, I suppose, but it’ll puzzle old Clear-Orf,” said Fatty, blotting the envelope. “Well, now I’ve taught you to write in invisible ink. Simple, isn’t it?”

“Awfully,” agreed Pip. “But I don’t quite see what use it will be to us, Fatty.”

“You never know,” said Fatty. “One of us might be captured in one mystery we solve, and we might want to get a message to the others. If we wrote it in invisible ink our enemies wouldn’t be able to read the message.”

Bets thought this sounded rather thrilling, though she didn’t very much want to be captured. Then a thought struck her.

“We’ll all have to carry an orange about with us, if ever we have enemies,” she said. “Won’t we? We’d better not take very juicy ones, or they’ll get squashed.”

“And we’d have to take a pen,” said Pip. “Well, I shan’t bother till we have enemies.”

“I shall,” said Fatty seriously. “You never know when you might need to write an invisible message. I take tons of things about with me in my pockets, just in case I might need them.”

This was quite true. The others were often amazed at the things Fatty carried about with him. As a rule he had practically anything needed in an emergency from a lemonade-bottle opener to a pocket-knife that contained twelve different kinds of tools.

“My mother goes through my pockets each night and won’t let me keep half what I want to,” said Pip.

“My mother never does things like that,” said Fatty. “She never bothers about my pockets.”

The others thought that it wasn’t only Fatty’s pockets his mother didn’t bother about - it was Fatty himself! He seemed to come and go as he pleased, missed his meals if he didn’t want them, went to bed what time he liked, and did more or less as he wanted to.

“Fatty, you said you’d show us how to get out of a locked room if the key wasn’t on your side,” said Bets, suddenly remembering. “There’s time to do that, too. Will you?”

“All right,” said Fatty. “Take me up to one of your box-rooms, where I shall be out of the way. Lock me up, and leave me there. Come down here, and I’ll join you in a few minutes.”

“Fibber,” said Larry and Pip together. It really did sound quite impossible.

“Well, try me and see,” said Fatty. “I don’t usually say I can do things if I can’t, do I?”

In excitement the children took Fatty upstairs to a big boxroom, with bare boards inside it, and on the landing as well. They put him inside, then turned the key in the lock. Larry tried the door. Yes, it was well and truly locked.

“You’re locked in, Fatty,” said Pip. “We’re going down now. If you can get out of here, you’re clever! You can’t get out of the window. There’s a sheer drop to the ground.”

“I’m not going to try the window,” said Fatty. “I shall walk out of the door.”

The others went down, feeling rather disbelieving. Fatty surely couldn’t be as clever as all that! Why, it would be like magic if he could go through a locked door!

Only Bets really believed he could. She sat with her eyes on the playroom door, waiting for him to come. Pip got out the ludo board.

“Let’s have a game,” he said. “Old Fatty won’t be down for ages, I expect. We shall hear him yelling to be let out in about ten minutes’ time!”

They set the counters in their places. They found the die, and put it in the thrower. Daisy threw first - but before she could move her counter, the door opened and in walked Fatty, grinning all over his plump face.

“Golly! How did you do it?” asked Larry, in the greatest surprise.

“I knew you would!” squealed Bets.

“How did you do it?” asked Pip and Daisy, burning with curiosity. “Go on - tell us.”

“It’s easy,” said Fatty, smoothing back his tidy hair. “Too easy for words.”

“Don’t keep on saying that! Tell us how you did it!” said Larry. “It’s extraordinary.”

“Well, come up and I’ll show you,” said Fatty. “As a matter of fact, it’s a thing all detectives ought to jolly well know. Elementary.”

“What’s elementary?” asked Bets, climbing the stairs behind Fatty.

“What I’ve just said - too easy for words,” said Fatty. “Well, here we are. Now, Larry, you lock us all four into the room - Buster too, if you like, or he’ll scratch the door down - and then you can all watch what I do. I tell you, it’s elementary!”

The three who were locked in with Fatty watched in excitement. They saw the door shut. They heard Larry turn the key in the lock. They each tried the door. Yes, it was locked all right.

“Now watch,” said Fatty. He took a folded newspaper from his pocket and unfolded it. He flattened the big, wide double-sheet. Then, to the children’s surprise, he slid the newspaper under the bottom of the door until only a small piece was left his side.

“What have you done that for? That won’t open the door!” said Bets. Fatty didn’t answer.

He took a piece of wire from his pocket and inserted it into the keyhole. The key was in the other side, where Larry had left it. Fatty jiggled about with the piece of wire, and then suddenly gave a slight push.

There was a thud on the other side of the door. “I’ve pushed the key out,” said Fatty. “Did you hear it fall? Well, the rest is easy! It’s fallen on to the newspaper outside - and all I have to do is to pull the paper carefully back - oh, very carefully, - and the key will come with it!”

Holding their breath, the children watched the newspaper being pulled under the door. There was a fair space between door and boards, and the key slid easily under the bottom of the door, appearing inside the room!

Fatty took it, slid it into the lock, turned it - and opened the door!

“There you are!” he said. “Very simple. Too easy for words! How to get out of a locked room in one minute!”

“Fatty! It’s marvellous! I’d never, never have thought of that!” cried Daisy. “Did you make up the trick yourself?”

Much as Fatty liked the others to think he was marvellous, he was too honest not to admit it wasn’t really his brain-wave. “Well, I read it in one of my spy books,” he said, “and I tried it out when I got locked in for a punishment one afternoon last term. It gave the master a turn, I can tell you, seeing me walk past him after he’d locked me up.”

“It’s wonderful,” said Bets. “So easy, too. There’s only one thing, Fatty, though - it wouldn’t work if you were locked up in a room that had a carpet going under the door, because there wouldn’t be room to pull in the key.”

“You’re right, Bets. That’s a good point,” said Fatty. “That’s why I wanted to be locked into a boxroom, and not in the playroom downstairs.”

The others were so thrilled with this new trick that they wanted to try it themselves.

“All right,” said Fatty. “It will be good practice. You simply never know when you might be locked up somewhere. Each of you do it in turn.”

So, much to Mrs. Hilton’s surprise, the five children and Buster spent the whole afternoon apparently doing nothing but walk in and out of the cold boxroom, to the accompaniment of squeals and giggles.

“Jolly good, Find-Outers,” said Fatty, when even Bets could escape from the locked room quite easily. “Jolly good. Now tomorrow I’ll go up to London and get some disguises. Look out for some fun the day after!”