Liquid helium-3 and helium-4 are remarkable substances. They are quantum liquids, meaning that their behavior is governed by the laws of quantum mechanics. Because of their small atomic mass, each isotope exists in a liquid state down to the temperature of absolute zero. And at sufficiently low temperature, each becomes a superfluid. However, the two isotopes have very different properties because 3He is a fermion and 4He is a boson. As a result of their different statistics, superfluidity in 3He appears at a temperature one-thousandth of that at which superfluid 4He forms. A second difference is that 3He has multiple thermodynamic phases. © 2018 American Institute of Physics Inc. All rights reserved.