A father was trying to teach his young son the evils of alcohol. 

He put one worm in a glass of water and another worm in a glass of whiskey.

The worm in the water lived, while the one in the whiskey curled up and died. 

"All right, son," asked the father, "what does that show you?" 

"Well, Dad, it shows that if you drink alcohol, you will not have worms."