Density is how much of something is packed into a given space. A bowling ball and a basketball might be similar in size, but the bowling ball has much higher density because it contains more matter squeezed into that space.

Scientists measure density by comparing an object's mass (how much stuff it contains) to its volume (how much space it takes up). Iron has high density: a small iron cube feels surprisingly heavy in your hand. Styrofoam has low density: a large block of it weighs almost nothing because most of its volume is air.

Density explains why some things float while others sink. Objects with lower density than water, like wood or inflatable pool toys, float on top. Objects with higher density than water, like rocks or metal spoons, sink straight to the bottom. Ice floats because water expands when it freezes, making ice less dense than liquid water.

The word also describes how closely things are packed together in other contexts. A densely populated city has many people living in a small area, while a sparsely populated area has few people spread far apart. A dense forest has trees growing close together, making it hard to see through. Sometimes people describe a difficult book as dense, meaning it packs lots of complex ideas into every sentence.