Question

Why do we laugh at a pratfall?  Why do we laugh at TV shows that feature people falling off sleds, bicycles, horses, boats, and every other imaginable conveyance?  That feature men clutching their groins in agony after getting injured in in a variety of stupid situations?  That show women and children screaming in fright from practical jokes?

What is it in us that delights at another person's pain or embarrassment?