‘It’s amazing, Mum!’ she said. ‘This ship weighs over two thousand tons and is operated from this one tiny place. Did you know everything is lighter in water?’

‘That’s Archimedes’ principle,’ said Catrin.

‘You what?’

And suddenly, without warning, Catrin tapped into the archive of her science-trained mind and speedily reeled off the definition she’d learned several decades earlier: ‘Archimedes’ principle states that the upward buoyant force exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the centre of mass of the displaced fluid.’

Lana and Romy sat stunned and open-mouthed for a moment, before Lana began slow-clapping her brainy friend.

‘Way to go, Cat, way to go!’

‘Mum, you’re such a swot!’

‘It’s why boats stay afloat and why, when you’re in the sea, you can carry someone much heavier than you. Don’t you remember you used to do it with your dad when you were little?’

‘Umm …’

‘You used to think it was magic that you could lift him up in the swimming pool and carry him around as if he was light as a feather!’

There was a beat of sadness. One of many that had occurred over the preceding couple of days, and surely there would be more to come – they were unavoidable, of course. But at least on board the Princess Eugenie the environment was all shiny and new, and a world that neither Cat nor Romy had ever visited before. It was a zone that was free from memories of Sol.

On Christmas morning, Catrin awoke early. For the first few seconds of consciousness, she forgot that Sol had died. This happened almost every day. She looked round at Lana, who was out for the count, and Romy, who was snoring in her bunk. She sighed, clambered quietly out of bed and wrapped a blanket around her shoulders before heading out on to the balcony, where she sat on the little deck bench and stared out to sea. The morning was fresh and misty but she could just make out the shape of land in the distance. She tried not to cry – she so didn’t want to cry. When was the crying ever going to stop?

A few minutes later she heard the door slide open and was joined by Lana, who stumbled out on to the balcony for her early nicotine fix.

‘Budge up,’ she said, lighting her cigarette and clambering under the blanket with her sad friend.

‘You stink,’ said Catrin in an attempt to lighten the mood.

‘You don’t smell so great yourself,’ said Lana, puffing away, regardless of how much Cat hated it. ‘D’you wanna drag?’

‘Lana, you of all people know that I have never smoked in my life. Why on earth would I start now at the age of fifty?’

‘Because it’s Christmas Day, and your heart is breaking, and I’m trying to distract you by being incredibly irritating.’