question of the day

"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.