 Oxygen is one of the most important elements on Earth. Without it, humans wouldn't be alive. But it's not only important to us because it's in the air that we breathe. Scientists have also found ways to use oxygen to help us understand Earth's past climate. We can do this because not every atom of oxygen is exactly the same. There are three different isotopes of oxygen which occur naturally on Earth. In its most common form, oxygen has eight protons in its nucleus and eight neutrons. All oxygen atoms have eight protons, but the amount of neutrons can vary between individual oxygen atoms. These variations in neutrons change the atomic weight of the oxygen atom and are called isotopes. Changing the number of neutrons in an atom does not change the element. It's still oxygen, but it does change the weight of the atom so that it behaves slightly differently. For my work, I'm looking at differences in the amount of one isotope of oxygen to another in water molecules. As a quick recap, water has a chemical formula H2O, which means it has two hydrogen atoms and one oxygen atom. The oxygen atom has three different isotopes. The most common is 16 oxygen, which makes up over 99% of all oxygen on Earth. About 0.2% is found as 18 oxygen and an even smaller amount is found as 17 oxygen. Oxygen isotopes in water are important because small changes in the amount of one oxygen isotope compared to another are driven by changes in temperature and humidity. So by measuring the amount of the different isotopes, we're able to reconstruct changes in Earth's climate system through time.