The Joke Between Caine & Weisz

I watched this movie last night and had to rewind to listen to the joke again but i couldn't for the life of me understand what Michael Caine says that makes them both laugh.

I'm referring to the scene after Weisz's character has a nightmare and she tells Caine's character that she's good in bed and he says,' I know.'
And she says,' How do you know?'

Then he says something about 'between the sheets'. Did ANYONE hear what he said? I was dying to find out as it's a genuinely nice moment the two of them have and i think the only time we see the two of them laugh together and i feel like i'm missing out by not knowing what he said

HELP ME!

Thanks


Youth - 10/10