Density compares mass to volume, which explains why a small rock can feel heavy.

The longer version

Density is the amount of matter packed into a given space, found by dividing mass by volume. A pebble can feel surprisingly heavy because the minerals in rock pack a lot of mass into a small volume, while a much larger piece of foam contains mostly air. Size alone says nothing.

This explains why some large objects float and some small ones sink. What matters is how the object's density compares with the fluid around it, not its weight by itself. Ships of steel stay afloat because their hulls enclose a great volume of air, lowering the average.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.