Little Johnny and Viagra

Viagra
At school little Johnny learns about medicines.
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 "
She gasped .. but taking control of herself and trying to look normal she asked ... "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, may be your shit will get harder" ...