Why does water expand when it freezes, while almost every other liquid contracts? If ice didn't float, how would that single quirk change the entire history of life on Earth?
The Turritopsis dohrnii (the "immortal jellyfish") can revert its cells back to childhood when it gets old or damaged, effectively restarting its life cycle. Why haven't complex mammals evolved a similar trick?
Why is the speed of light approx 3 times 10^8 m/s the absolute speed limit of the universe? What physically stops an object with mass from moving faster?