My just about year-and-a-half-old son made his first joke yesterday. "Pizza block." For reference, "block" is what he calls sunblock, and pizza is his favorite food. He said "pizza block" and chuckled uncontrollably at his delightfully ironic play on words, which I took to mean "What's the deal with pizza? Sure, it tastes great, but it wouldn't make very good sunblock!"
I wonder if Jerry Seinfeld got his start this early.