Drop a lemon and a lime into a glass of water and something odd happens: the lemon sinks straight to the bottom, while the lime bobs at the surface. Both fruits feel about the same weight in your hand, but their insides are surprisingly different.
A lemon has dense, juicy flesh and a thicker rind, which together make it just slightly heavier than water. A lime has more tiny air pockets in its flesh and a thinner skin, making it just slightly lighter than water. The difference is small, but it is enough to flip them from “sink” to “float.”
This sink-or-swim trick is a fun way to see density in action. Density just means how much stuff is packed into a space. Cargo ships full of heavy steel float for the same reason limes do - they are full of air inside, so the whole thing weighs less than the water it pushes aside.
