Tommy was having pains in his knees so went to the doctor.

After examining him the doctor said "You've got kneeitis, it's caused by too much bending of the knees. I suggest you avoid climbing the stairs because it puts a terrible strain on the knees".

A month later, Tommy went bck to the doctor who examinied him again and told him that he'd recovered.

"Can I climb the stairs now?" Tommy asked.

"Certainly" the doctor replied.

"Phew, thank goodness" Tommy replied "I was getting fed up with climbing the drainpipe every time I needed to go to the bathroom."