Tommy was suffering from pains in his knees, so he visited the Doctor.

'You're suffering from a disease that we medical experts call "kneeitis", diagnosed Doctor Donaghue. 'Take it easy for a month or so and above all don't climb any stairs. That puts a terrible strain on the knees.'

A month later Tommy returned and after a brief examination was found to have recovered completely.

'Can I climb the stairs now Doctor?'

'Yes, no problem Tommy,' replied Donaghue.

'Thank Heavens,' uttered Tommy, 'I was getting a bit fed up with climbing up the drainpipe every time I wanted to go to the toilet.'