Pure, liquid water is most dense at 39.2°F (3.98°C) and decreases in density as it freezes. The water molecules in ice are held in a relatively rigid geometric pattern by their hydrogen bonds, producing an open, porous structure. Liquid water has fewer bonds; therefore, more molecules can occupy the same space, making liquid water denser than ice.

How many organisms have lived on the Earth since life began continues to be a very controversial subject. Some scientists believe more than two billion species have lived on our Earth over time, including those living today. In fact, some scientists estimate that about 90 to 99.9 percent of all animal and plant species that have ever lived on our world are now extinct. There are reasons why this number is difficult to pin down, including the fact that much of early life—especially those with soft bodies—left no trace. In addition, many of the fossils that exist are buried deep into the ground or have been weathered away by natural physical processes (for example, glacial or water erosion).