Eternity

I am not sure but I think it was in the movie "Woman in the Wind" (1990) starring Colleen Dewhurst that I heard this tale about forever.

Imagine a butterfly beating it's wings against a steel ball for so long that the wings make an impression in the ball.

Now imagine all the grains of sand on all the beaches of all the world.  Imagine each grain as a steel ball. 

By the time that butterfly has made a tiny dent in each of those balls, eternity will have just begun.