Alcohol
"Now, class, closely observe the worms," said the teacher while putting a worm into the water.
The worm in the water writhed about, happy as a worm in water could be.
He then put the second worm into the whiskey.
It curled up and writhed about painfully, then quickly sank to the bottom, dead as a doornail.
"Now, what lesson can we learn from this experiment?" he asked.
Johnny, who naturally sits in back, raised his hand and wisely, responded confidently, "Drink whisky and you won't get worms."