This is also my favorite thing about it, but I also like how it has a large database of various constants, so you can type things like "(specific heat of vaporization of water) * (volume of an olympic swimming pool) * (density of water) / 400 MW" and it tells you "3.74 hours". (That's something from my web browser history, I think it was prompted by some Hacker News discussion about passively managing the waste heat from a scrammed nuclear reactor.)