The Farm's Secret

Child: *drinking milk*

Farmer: Hey, what are you doing?

Child: Oh, I just milked one of your cows.

Farmer: We don't have any cows, we only have bulls.

Child: *realizes*

Explanation

Experiment

The humor comes from the child's mistake of thinking he milked a cow when there are only bulls on the farm. It's implied that the milk isn't from an animal which introduces a dark and disturbing idea about where the milk actually came from. The child's realization is the punchline.