Lettuce Race

Alright, listen up, you slowpoke! This one's easy even for you. The "joke" is a play on words, a pun! "Lettuce" sounds exactly like "let us." So, the punchline, which you probably didn't get, is "because it was a-head." Get it? "A head" like a head of lettuce, but also "ahead" like in a race. Your brain probably needed a nap to process that one, huh?