Thirty-One Knots

At eight forty on Saturday morning, Frida found a rope hanging from the diving platform. It had far too many knots.

She stood on the wet tiles of the rooftop pool and touched each knot with one finger. There were thirty-one of them, tied tight, and some were as big as plums. Behind her, the water looked grey and cloudy.

"Aarav, come and look at this," she called.

Aarav came out of the changing room with a mop over his shoulder. He was sixteen, like Frida. He worked at the pool on weekends to save money for a new bike.

"That is not a normal rope," he said. "Who tied all those knots?"

"I think the manager did." Frida held up a piece of paper. "Read this. It was on the front desk."

The note said, "Gone to collect the trophies. Back at ten thirty. Please start the water pump by nine thirty. The code for the pump box is on the rope."

"On the rope?" Aarav read the note twice. "Why didn't he just write the code down?"

"Last week he did, on a sticky note. A little boy found it and pushed the emergency button." Frida looked at the big clock above the snack bar. "The whole pool stopped for an hour."

"So now the code is hidden." Aarav put down his mop. "How long do we have?"

"Fifty minutes. The first swimmers come at half past ten. The water needs an hour of cleaning before they jump in."

"Then we need that code," Aarav said, and he started to pick at the nearest knot.

The knot was wet and hard. After three minutes, Aarav had only loosened it a little, and his fingers hurt.

"This will take all day," he said. "There are thirty more."

"Maybe the manager hid a paper inside one of them," Frida said. "Keep going. I will try the next one."

They worked side by side for ten minutes. Aarav opened four knots, and Frida opened three. Inside them there was nothing at all, only damp, twisted cord.

"Maybe we should cut the rope," Aarav said. "The scissors are in the first-aid room."

"If we cut it, we might destroy the clue." Frida wiped her hands on her shorts. "Let me check the box first."

The pump box stood in the corner of the roof, a grey metal cabinet with a heavy padlock. The lock had four number wheels. Above each wheel, somebody had stuck a tiny coloured dot. The first was blue, the second white, the third green, and the fourth yellow.

Frida stared at the dots for a long moment. Then she looked back across the roof at the rope, and her eyes opened wide.

"Aarav! What colour is the rope?"

"Grey, I think." He frowned. "No, wait. It is blue near the top."

They ran back to the platform together. Frida lifted the rope and walked her hands down it. The top part was blue. Further down it turned white, then green, and the last part was yellow.

"It is four pieces of old lane rope, tied together," she said. "The manager never throws anything away. He has been saving these pieces for years."

"And the dots on the lock have the same four colours." Aarav smiled for the first time that morning. "So we don't have to open any knots."

"We only have to count them. Seven knots might mean the number seven on the first wheel."

"That is a good guess, but we have to be careful," Aarav said. "If we make one mistake, the lock will not open."

Frida checked the clock. It was nine fifteen. She counted the blue part first, touching each knot slowly. Aarav counted the white part, and then they swapped places to check each other's numbers.

"Blue has seven," Frida said. "White has nine."

"Green has eight," said Aarav. "And yellow has six."

They ran to the pump box and tried seven, nine, eight, six. The padlock did not move. Aarav pulled it hard, but it stayed shut.

"We made a mistake," he said. "Think, Frida. What do we know for sure?"

"We know the total." She stopped. "There are thirty-one knots in all. I counted them at the beginning. But seven, nine, eight and six make only thirty."

"So one of our numbers is wrong, and we only lost one knot." Aarav looked at his watch. "It is nine twenty-one."

"Which part is the hardest to count?"

"The yellow part. It is full of tiny knots, and the end of it is dirty from the floor."

They hurried back to the rope. This time Aarav closed his eyes and counted the yellow part with his fingertips. Halfway down, he stopped and made a small noise.

"Frida, feel this. These two are tied right next to each other. Last time they felt like one big knot."

Frida touched the place and nodded. "That is the missing one. Yellow has seven, not six."

The clock said nine twenty-six. They ran across the wet tiles to the pump box. Frida turned the wheels with shaking fingers: seven, nine, eight, seven.

The padlock opened with a loud click.

Inside the cabinet there were two black switches and one big green button. A paper sign said, "Switches up. Then press." Aarav pushed both switches up, and Frida pressed the button with her whole hand.

For a second, nothing happened. Then a deep hum started under the floor. At the edge of the pool, the water began to move, slowly, towards the filter. Small bubbles rose up and popped.

"Nine twenty-nine," Aarav said, and he dropped onto a plastic chair. "One minute to spare."

"The water will be clear by ten thirty." Frida laughed. "And the swimmers can come in."

Aarav took out his phone. He had sent a message to the manager at nine ten, and there was still no answer. Now a new message arrived, and he read it aloud.

"It says, 'Did you find the code? I tied the knots in the car park at midnight. It took me an hour.'"

"An hour!" Frida shook her head. "He should have used fewer knots."

"I agree. Thirty-one is a crazy number." Aarav grinned. "But the colours were clever."

Frida went to the front desk and found a pen. She turned the manager's note over and wrote in big letters, "Next time, please use fewer knots." Then she stuck the note on his office door.