Heraclitus: Time is like a river flowing downstream. And you cannot step in the same river twice.

Mrs. Heraclitus: Don't be an ass, Heraclitus. Of course you can step in the same river twice. All you have to do is go downstream at the same rate that the river is flowing..

Heraclitus was amazed! He ran down to the river Styx and put his foot in. Then he ran downstream and put his foot in again and he ran downstream and put his foot in again, and again, and again, until he reached the Aegean Sea and he drowned.

(With thanks to Severn Darden)