"Mom, how do they know that infinity is the biggest number?"
She's only just turned 7.
A lively discussion ensued, involving the almost-9 and almost-5-year-olds as well.
UPDATE: Tom gives a mini-lecture in the comments. Here's a paraphrase of my answer (since I can't remember my exact words): "Infinity is not a number, it's an idea." That was the basis of the lively discussion.