Heh, I love being around geeks.
Me: "Hmm… the instructions on this frozen lunch are for a 1000W microwave"
Co-worker: "Well, a cup of water takes 2m20s to boil. You could work it out from there."
Me: "Hey! You're right! Assuming the water is at 10℃ when it goes in, that means the microwave is outputting… 672W of power." (Yes, it's an old 800W microwave)