Why did the dinosaur cross the road?
Because the chicken hadn't evolved yet.
My kids also love "the duck joke." They've heard me tell it so many times even the 4 year old can now tell it himself.
A duck walks into a bar and asks, "Got any grapes?" The bartender says, "Nope," and the duck says, "Okay," and leaves. But the next day the duck comes back and asks, "Got any grapes?" The bartender says, "No! I already told you! This is a bar! I don't have any grapes! Get out of here!" So the duck leaves. But the next day, he comes back again and asks, "Got any grapes?" The bartender yells, "For the last time, no! I do not have any grapes! If you come in here one more time asking for grapes, I'm going to nail your stupid webbed feet to the bar!" The duck says, "Okay," and leaves. But the next day, he comes back again! He walks up to the bar and asks, "Got any nails?" The bartender says, "No!" The duck says, "Well, in that case - got any grapes?"