re: What's the simplest thing you've had to explain to a full grown adult?

My wife was changing my infant son's diaper when he was around 1-2 weeks old (she had a C-section and was bed ridden for a while) and noticed his balls were quite loose and deflated. Previously she had only changed him when he was cold, I guess, and they had been tight. She freaked the frick out thinking something was very wrong.

I had a humorous conversation with her about male anatomy and natural thermometers.