What's a fact you learned from QI that sounded completely made up until you checked it yourself?

One of my favourite things about QI is when an answer sounds so obviously wrong that you're convinced they're setting up another joke, and then it turns out to be completely legitimate. Those are usually the facts I remember much longer than the straightforward ones because I've ended up reading about them afterwards.

What's the best QI fact that made you think there's absolutely no way that's true the first time you heard it?