Anyone who's watched America's Funniest Home Videos knows getting kicked in the nuts is hilarious. But that dad whose toddler just whacked him with a five iron is writhing on the ground for a reason. While vital organs like the heart and brain are protected by bone, testicles hang outside the body to stay cool enough for sperm production. By equipping them with oodles of nerve endings, evolution has guaranteed that we'll go to great lengths to defend them. Hence, a guy's hands fly to his boys -- and the boys retreat to the groin -- when he suffers even a glancing blow. Should he take a harder shot, here's a hint of what it feels like.
How well do you know his crotch? Take the quiz to find out if you're master of his domain.