What's three foot high, stands at the foot of the bed, and takes the piss out of you?

A dialysis machine.

After 5 1/2 years on dialysis I feel entitled to tell that joke, but I think I would consider it tasteless in the extreme for anyone else to tell it in my presence. On occasions very close friends have told it to others in front of me, and the reaction, if the others know about my circumstances, can be fascinating. They generally look at my face and if my reaction seems favourable they laugh nervously. If I'm feeling particularly evil I feign indignation and remonstrate with the teller. Everyone gets very uncomfortable until we let them know we're winding them up.

I suppose the moral is some jokes are better left untold unless you know everyone extremely well.