Density is the ratio of an object’s mass to its volume. Each material has a different density, the difference depends on how closely the molecules in the material are packed together.

The range of densities in the universe is amazing. At the high side are black holes (said to be infinitely dense) and pulsars. A thimble full of pulsar would weigh as much as 50 million elephants. Air is not very dense but still has 10^24 atoms per cubic meter. Interstellar space has a mere 500,000 atoms per cubic meter and the space between gallaxies is thought to have a few atoms per 10 cubic meters. Ratios of density in the universe spans 10^44.

If we make the density of freshwater at sea level equal to 1, then frozen water (ice) has a density less than 1 because it floats (btw, if ice didn’t float, it is possible that life on earth wouldn’t exist, see Ice Ice Baby). Boats float because they have an average density of less than 1 – which is interesting because boats are made mostly of stuff with a density much higher than 1 (see How do big steel ships float?).

Rocky planets and asteroids have densities between 2 and 5. The earth’s core is thought to have a density of 12. The average density of earth is about a 5.5.

Hot air just doesn’t rise because it is hot, an equally important point is that cold air sinks because it is denser. This is how convection works.

Saturn has a density less than 1. So, a cup full of Saturn would float. The Sun’s density is about 1/4 of what earth’s is – a mere 40% more dense than water.

How dense are humans (physically not mentally)?  It varies by person. A big factor is fat content – fat floats while bone sinks and most organs and muscle are about the same density as water. When we fill our lungs, we are overall less dense than when we empty our lungs.  The average density of the human body is 985 kg/m3, and the typical density of seawater is about 1035 kg/m3 while freshwater is 1,000 kg/m3.