Density is a measure of a substance's mass relative to the amount of space it takes up. Dense substances, like iron and other metals, are massive -- they weigh a lot but take up very little space. Gases, in contrast, take up a great deal of space and weigh very little.

Density is a measurable characteristic of all substances and is generally recorded in grams per cubic centimeter. Iron, for example, has a density of 7.9 g/cm³. By comparison, the density of oxygen -- at sea level and a temperature of 20 degrees Celsius (C) -- is 0.0013 g/cm³.

For many substances, including most solids, density varies only slightly with changes in temperature. The density of gases, and to a lesser extent that of liquids, changes as ambient temperatures rise and fall. The molecules of these substances generally become more active and more spread out -- less dense -- as temperatures rise. Conversely, they become more tightly packed as temperatures decrease.

Water follows these general rules, but with two exceptions that are important to life on Earth. First, water is less dense as a solid than as a liquid. This is because of the way water molecules arrange themselves when they form ice crystals. The crystalline structure of ice holds molecules at greater distances from one another than when they are in a liquid form. This phenomenon explains why ice is less dense and floats on water. If this weren't the case, lakes, ponds, and oceans, even in temperate parts of the world, would likely freeze permanently.

Second, water is most dense at about 4 degrees C. As it warms up from 0 degrees and begins to near the 4-degree mark, it becomes increasingly dense. Then, like most other substances, it becomes less and less dense as temperatures continue to rise. In spring and fall, temperature changes cause the densest water at the surface of a lake or pond to fall to the bottom, where it replaces warmer or cooler water. This causes nutrients that have settled at the bottom and oxygen concentrated near the surface to become more evenly distributed throughout a lake or pond, thus making them more available to the aquatic organisms that need them.