Celina picked up on the Doctor's tells really quickly. It wasn't that hard, the Doctor really wasn't that subtle, plus,

if there was one thing she paid attention to, it was signs that she, or others, needed to pee. It also only took some math to find out how often he would need it. The Doctor was also perfectly blunt with her, as she did not judge him for natural bodily functions. After all, when you've gotta go you've gotta go. Time Lords included.

The Doctor looked out of the window at the desert, and she did the same. It seemed like an ordinary, safe desert on her side of the bus. Which was good, since the Doctor had ignored the Queen of England's suggestion that he use the loo before he left, so according to her count, he had about 12 hours till his need would get urgent, and she knew they'd definitely be stuck on this planet for at least six of them. Celina thought it was nice that it was a desert planet, since she'd seen Tatooine in Star Wars, so knew she had little to worry about. Plus, it meant the Doctor could pee in the desert later.

The bus needed to get its engine de-sanded and its wheels free of the sand, so the Doctor took the opportunity to pop off for a wee. Only Christina followed him. Celina noticed and intervened immediately.

"Yes you are. He said "back in two ticks", that means don't follow him. Capisce?" she said, seriously, blocking her path.

"I need to make sure he doesn't run off. What's the problem?" Christina replied, walking past her.

"Where would he run off to?" Celina asked, remembering the TARDIS as the answer, had it not been parked at Buckingham Palace. "We're on a desert planet with a broken bus, surrounded by nothing but sand. And why can't you wait two minutes?" Celina asked.

At this point the Doctor had heard the two women, and turned around. Celina rolled her eyes, indicating Christina, giving him a look of sympathy.

"Yeah, why can't you?" the Doctor asked, confused.

"I don't know you. What's wrong with wandering with company?" Christina asked.

The Doctor looked to Celina to explain.

"Seriously? Come on. Think about what he just said."

Lady Christina did not get the hint.

Sighing, Celina whispered to Christina "he needs to 'water the sand',".

"Oh! Oh, I'm so sorry. Of course. By all means," she said, understandingly, walking back to the bus.