Or an even more simple rule which should generally apply anyway: do not disrespect other people? Show some consideration for others.
A joke is a joke if everyone finds it funny, that is including the person on the receiving end. I love and respect my Grandad, he has worked very hard in his life and is a very good man, so why is it funny that he is compared to Michael Jackson? If you find that funny then you have a rather shite sense of humour to be honest. I suppose I shouldn't expect any better really, but I always hold out some hope. It's a shame I am disappointed every time.