What milestone did your children hit that you weren't ready for?

I'm a father of two girls, ages are 6 and 4. Miss 6 has had a wobbly tooth for a few weeks now (the way she has harped on it felt like years) now last week it came out. Being that it was loose for so long I thought I had come to terms with that, I hadn't. It's odd but it shook me harder than I would have expected.

What milestone has your little one hit that you weren't ready for? prepare this foolish dad for more things to come!