Lighthearted: Have you only just realised something is infact not what you thought?

[coffeehabit]

Didn't quite know how to title this but:

I have always thought the second hand on a clock was, like, first and second. Only just found out it means the seconds..... I'm 52.....

What seemingly obvious things have you only just realised are infact not what you thought?

I used to think scaffolding was to hold up unstable buildings. I was terrified of it .