Mr. Bean and Einstein

Mr. Bean and Einstein were taking a coffee when Einstein challenges Mr. Bean to a knowledge challenge.

**Einstein**: here is the deal, Bean. I'll make you a question. If you don't know the answer you give me 1$. Then you ask me something and if I have no answers for you, I'll give you 1000$

**Mr. Bean**: Fine.

Einstein, sure of his upcoming success, proceeds to ask Bean his question.

**Einstein**: What's the basic of quantum physics?

**Mr. Bean**: ehm...
He gives Einstein $1.

**Einstein**: alright, it's your turn now.

**Mr. Bean**: What has four legs while crossing the street, five legs when sitting down and two legs while flying?

Einstein thinks once, twice... But he really can't come up with an answer so he gives Mr. Bean $1000.
But then...

**Einstein**: But what was the answer?

*Mr. Bean gives Einstein $1*.