The lady teacher asks the pupils what kind of medicines they know and what they are used for.

The first pupil said: Tylenol?

Very good! And what is it used for?

It is used for headache.

The second pupil said: Nytol

Excellent. And what it is used for?

To help you sleep

Now it is Johnny's turn and he said: Viagra

Johnny. What is it used for?

I think it can be used for diarrhea.

Who told you this?

Nobody, but every evening my mother tells my father 'take a Viagra, maybe that little shit will get harder'.