What If?2

This lets us apply one of my favorite weird equations, which says that the edge of a spinning disk can’t go faster than the square root of the specific strength2 of the material it’s made of.

There’s a neat way to answer “Will it float?” questions without doing too many complicated calculations. Water is roughly 1,000 times denser than air,10 so if you want to know whether something could float if you filled it with helium, just estimate how heavy it would be when filled with water, then move the decimal point over 3 places. That’s how much buoyancy it could produce, so it’s how light the solid parts have to be in order to float.

All the clouds in the world, combined, hold about 13 trillion tons of water. If all that water were spread out evenly and fell at the same time, it would cover the Earth with an inch of rain—or a foot of snow.