The probability of an interaction between an incoming particle and a target particle, e.g. between a passing slow neutron and a uranium-235 nucleus. Should two hard inelastic spheres collide, their cross-section is proportional to their geometric size. In collisions of particles that can interact at a distance (e.g., by electrostatic, gravitational, or nuclear force), their cross-section is generally larger than their size. Furthermore, one must distinguish between the probability of any interaction and the probability of a particular type of reaction (e.g., a reflection at a certain angle); the probability of a particular reaction is always lower than the probability of any interaction. For the design of a fission reactor, it is important to know the cross sections between the particles that will interact. For example, slow neutrons have a higher probability of inducing fission of a uranium-235 nucleus and are therefore slowed down by a moderator. Boron has a high cross-section when interacting with neutrons, and is thus a good absorber of neutrons.