Viagra for diarhea

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.