Most jokes I've heard 2YO's make up, especially boys, go like this:
Q: [insert almost anything here, like, "Why did the chicken cross he road?]
A: Because he was a poopy-head!!!
OK, here's one that still gets some mileage with my girls:
Q: What's green and goes 90 mph (or kph) backwards?
A: Sniff backwards loudly!

I also get applause for belching words.....
