Densa

Meet Densa, a block who knows exactly how much mass she packs into each bit of space.

Density is mass per unit volume, ρ = m/V. It tells you how tightly matter is packed in a material. A small iron block can outweigh a large block of wood because iron is much denser. Density also decides floating: an object floats in a liquid if its average density is less than the liquid's, an idea that goes back to Archimedes.

A cube of water 10 cm on each side has a volume of 1,000 cm³ and a mass of about 1 kg, a density of 1 g/cm³ or 1,000 kg/m³. The same cube made of iron, density about 7.87 g/cm³, has a mass of 1,000 × 7.87 = 7,870 g, about 7.9 kg.

Density helps identify materials, check the purity of metals, design ships that float, and explain why hot air rises and oil floats on water.

Density changes with temperature and pressure. Most substances get denser as they cool, but water is densest at about 4 °C, which is why ice floats and lakes freeze from the top down.