Puzzles

Here I opened this thread to know what puzzles are stored in your minds. Everyone who has one or more puzzle can throw it here and the one who will answer it right must bring his/her own next buzzle and throw it here. I'll start with a very simple one:
Born walking on four
Grown walking on two
When old, walking on three

A human! That wasn't too difficult.

Here is an easy brainteaser:

A man, when asked by his grand-daughter, as to how old he was, replied, "My age is now four times yours, but five years ago it was five times yours." How old was he?

5 years ago the grandfather was 75 ys and she was 15. Now he is 80 yr and she is 20.

Now take this:
who is that person?
the father of your brother's son.