This one is clean enough to tell my grandmother:

A preacher was walking through Africa, when he comes across a Lion. He quickly kneels and starts praying for the lord to spare his life. He prays for what seems like for ever. He looks up and sees the lion praying too. 'Say, Mr. Lion, why are you praying?' 'I'm not, you're praying that I don't kill you. I'm saying grace before dinner.'

The rest of his act would make Andrew Dice Clay blush.