"Little Johnny on medicines" joke

The lady teacher asks the pupils what kind of medicines they know and what they are used for.
The first pupil said: Panadol?
Very good! And what is it used for?
It is used for headache.
The second pupil said: Piriton
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 diarrhoea.
Who told you this?
Nobody, but every evening my mother tells my father' take a Viagra, may be that little shit will get harder'.