The problem that it is very difficult to obtain exact solutions to systems involving interactions between more than two bodies—using either classical mechanics or quantum mechanics. To understand the physics of many-body systems it is necessary to make use of approximation techniques or model systems that capture the essential physics of the problem. For some problems, such as the three-body problem in classical mechanics, it is possible to obtain qualitative information about the system. Useful concepts in the quantum theory of many-body systems are quasiparticles and collective excitations. If there are a great many bodies interacting, such as the molecules in a gas, the problem can be analysed using the techniques of statistical mechanics.