Mass and Energy

The other day, shortly before bedtime, Nolan came trotting into my bedroom. He informed me he was really tired and he needed to see what he weighed. He messed around the with the scale for a minute ('tap on it, wait for the zero, then step on it'!). After a long pause he said, "That's funny. I still weigh 44. Even though I'm really tired."

I laughed and explained that your weight doesn't change even when you're tired. Nolan was a bit annoyed. That wasn't interesting to him at all. So he decided to weigh his wristwatch. He has quite the watch. He says it weighs four pounds.