Hyperons are subatomic particles of the class known as baryons. Like all baryons, they are composed of three quarks. The term hyperon is generally used for a baryon containing one or more strange (s) quarks, as opposed (for example) to the proton and neutron, which contain only up (u) and down (d) quarks. The strange quark being unstable, hyperons decay into lighter baryons (such as protons or neutrons) plus mesons, with typical lifetimes of approximately 1/10 of a nanosecond. At high energies, these lifetimes are sufficient for a hyperon to travel several meters before decaying, since the hyperon can be moving at very nearly the speed of light and thus experience the time-dilation effect of Special Relativity. This long decay distance makes hyperon-beam experiments (such as the HyperCP experiment) feasible.