consists mostly of two isotopes: 99.3% 0.7% Of these two, undergoes fission only by fast neutrons. About 0.7% of natural uranium is , which will fission by both fast and slow (thermal) neutrons. When the uranium undergoes fission, it releases neutrons with a high energy ("fast"). However, these fast neutrons have a much lower probability of causing another fission than neutrons which are slowed down after they have been generated by the fission process. Slower neutrons have a much higher chance (about 585 times greater) of causing a fission in than the fast neutrons. The common solution to this problem is to slow the neutrons down using a neutron moderator, which interacts with the neutrons to slow them. The most common moderator is ordinary water, which acts by elastic scattering until the neutrons reach thermal equilibrium with the water (hence the term "thermal neutron"), at which point the neutrons become highly reactive with the . Other moderators include heavy water, beryllium and graphite. The elastic scattering of the neutrons can be likened to the collision of two ping pong balls; when a fast ping pong ball hits one that is stationary or moving slowly, they will both end up having about half of the original kinetic energy of the fast ball. This is in contrast to a fast ping pong ball hitting a bowling ball, where the ping pong ball keeps virtually all of its energy. Such thermal neutrons are more likely to be absorbed by another heavy element, such as , or . In this case, only the has a high probability of fission. Although rarely undergoes fission by the fast neutrons released in fission, thermal neutrons (i.e. neutrons that have been slowed down by a moderator) can be captured by the nucleus to transmute the uranium into which rapidly decays into which in turn decays into . has a neutron cross section larger than that of , which means that in turn, it can absorb yet another thermal neutron. About 73% of the created this way will undergo fission from capturing a thermal neutron while the remaining 27% absorbs a thermal neutron without undergoing fission, is created, which rarely fissions with thermal neutrons. When Plutonium-240 in turn absorbs a thermal neutron to become a heavier isotope which is also fissionable with thermal neutrons very close in probability to Plutonium-239. These effects combined have the result of creating, in