A good conversation starter

and great all-around meme from Carl Sagan: 

Suppose your father or mother--let's say father for the sake of definiteness--walked into this room at the ordinary human pace of walking. And suppose just behind him was his father. And just behind him was his father. How long would we have to wait before the ancestor who enters the now open door is a creature who normally walked on all fours?

The answer is a week. The parade of ancestors moving at the ordinary pace of walking would take only a week before you got to a quadruped.