A membrane, or ’brane for short, is a multi-dimensional structure that perhaps exists in something like a supersymmetric bulk described above. Membranes can move around in the bulk—like a huge jellyfish floating and swirling around in a vast ocean—and interact with (that is, “bump into”) one another, possibly causing vast releases or exchanges of energy. A cosmological model that invokes ’branes is often called a membrane theory or M theory. These models propose the existence of different kinds of ’branes, which are given names like m-’branes, n-’branes, or p-’branes.