 Oxygen has three major isotopes, oxygen 16, oxygen 17, and oxygen 18. All of these isotopes have eight protons, but oxygen 16 also has eight neutrons, oxygen 17 has nine neutrons, and oxygen 18 has 10 neutrons. Because these substances have different weights, they behave differently in nature, and this allows environmental processes to fractionate them. By fractionation, I mean separation of the various isotopes by natural processes, and this fractionation allows geologists to use the isotopes as proxies. Here we consider the temperature proxy of oxygen isotopes. Oxygen 16 is much more abundant in nature than oxygen 18, but its mass provides for mass-dependent fractionation from oxygen 18, and this is a result of vibrational frequency differences that operate at different temperatures. For example, in the formation of calcite, CaCO3, warmer water will lead to inclusion of more oxygen 16 in the calcite molecule. In colder water, more oxygen 18 will be included in the calcite molecule. This allows us to use the relative ratio of oxygen 16 and oxygen 18 in the calcite of foraminifera as a proxy for paleo temperature.