the ancient Greek mathematician Archimedes discovered the principle of buoyancy while taking a bath. When he noticed the water level rise as he got in, he realized he could determine an object’s density by how much water it displaced. Legend has it he was so excited, he ran through the streets shouting “Eureka!”

And here’s a twist: ice floats on water because it’s less dense than liquid water—a rare trait among substances. Most materials become denser as they freeze, but water reaches its maximum density at about 4°C, then expands as it turns to ice. That’s why lakes freeze top-down, keeping aquatic life alive beneath the surface.