The cabin roof was pointed, but because Ursula leaned over while they were under sail, one side of the roof was almost flat.

“Okay, now put one of the cups in the middle of the bowl. Weigh it down with one of the fishing weights. Scoop up some sea water with the other cup and fill the bowl until it’s about an inch deep.”

Mia did all that, but she left out the weight for the cup. What was the point of that? She poured the sea water into the pan, and the cup started to float. Now Mia saw the point of the weight. She quickly rescued the cup before it toppled over into the salt water, and dropped the weight in.

“Now put the plastic sheet over the bowl loosely, so that it dips down in the middle. Use another weight to make sure it stays dipped, directly over the cup inside. Fix the sheet in place with a bungee cord around the rim …”

Eventually Mia had got it set up like Bear said.

“What does this do?” she asked.

“It’s called a solar still. The sun will warm up the sea water, so it will start to evaporate and leave the salt behind. It will condense on the plastic and turn back into liquid …”

“And trickle into the cup!” Mia realised happily.

“That’s right! It won’t get us much – maybe half a litre a day at most. But it should help us close the gap. That will get us five and a half litres out of the total six that we need. We’ll leave a tarpaulin out at night to collect any dew on deck – that’s another source of fresh water …”

Suddenly Mia barely heard Bear. There had been a flash of light from behind him. The sun reflecting off glass or metal. She ran to the back rail and stared.

“What is it, Mia?”

“There’s a ship behind us! Look!” She pointed to a tiny blob on the horizon.

Bear grabbed some binoculars. “Great spot, Mia! It’s an oil tanker. Fetch a flare from the locker, there. One with an orange top.”

The locker was full of metal tubes like batons. Mia grabbed the nearest one and passed it to Bear, but he shook his head.

“I don’t really want to let go of the tiller. You do it. Point it away from you, aim it up into the sky, and give that cord on the end a hard yank …”

Mia did, and she almost dropped the tube in surprise. It went off with a bang. An orange ball of fire shot into the air and floated down again like a mini sun.

“Shouldn’t we turn around towards it?” she asked.