Poor people have it,
rich people need it
and who does not eat it, dies.

I am what I am,
but if you know what I am,
I am not longer what I am.
Guess, what am I?

A woman lived in an apartment building on the seventh floor. Every morning she took the lift to the ground floor and went to her work. But when she came home, she took the lift to the fourth floor and then continued by walking the stairs up. Why would she do that?