Why do chickens sit on their eggs?

This is what Sam asked me.
I looked at him and hesitated. I had to think about it before I responded.
"It's so they can keep them warm." I told him.
"No, Mom. It's 'cause they don't have any chairs!"

Ha.

His first official joke and I don't know where he learned it. It made me laugh, and still does about every 3rd time he tells it to me:)

Thanks Sam, for the laugh.