The smart monkey.

Thomas goes to the zoo to feed the monkeys. He throws a
monkey a peanut, the monkey picks it up, sticks it in his rear,
pulls it out and eats it.
Thomas goes, "YUK!"
He throws the monkey another peanut. The monkey picks it up,
sticks it in his rear, pulls it out and eats it.
Thomas goes, "YUK!"
He goes to the zookeeper and he says, "Man, that is one very
stupid monkey."
The zookeeper says, "No, that's a very smart monkey. Last
week, somebody threw him a big peach, and he ate it, and he
couldn't pass the pit. So now he measures everything first."