I've started tellingjokes to my 3 yr-old. She sort of gets them but they have to be VERY simple. I picked up a book of kids jokes and looked at a couple of websites but most of the jokes are waaay too sophistated for her.
Current favourites of ours are:
What's very heavy and wears glass slippers? Cinderelephant.
Why did the elephant paint his toenails red? So he could hide in the cherry tree. (Took a lot of explaining and repeat performances but it was worth it.)
Ice cream and cats also figure heavily.
Dd's latest contribution was 'Why does a flower have a middle? Ans: Because it's a flower. Not rushing to book her as a stand-up just yet then....