Gavin Edwards, interviewing Randall Munroe for Rolling Stone:
I don’t know if this showed up in a What If answer, but it really surprised me that the pools at the bottoms of waterfalls aren’t hot. Heat is just kinetic energy, and I thought, okay, water’s falling this great height and has enough energy to run hydroelectric plants—there’s a lot of potential energy there. And water is heavy, and waterfalls are huge, so there’s a tremendous amount of energy being converted as the water falls from the top of Niagara Falls to the bottom, so the water on the bottom has to be pretty hot. I sat down and did the math: the mass of the water times gravity times the height, divided by the specific heat of water to come up with how many degrees the temperature increases per hundred meters. The temperature increase is barely measurable, and the reason is water just has an incredibly high heat capacity, it can soak up more heat per mass than anything except, I think, ammonia.
I love this man’s curiosity.