How do control rods work in a nuclear fission reactor?

In Nuclear Fission, neutrons collide with radioactive atoms (eg Uranium-235) in order to split the atom into two smaller atoms and releasing energy. The more neutrons there are the faster the reaction so to slow it down you need to remove the neutrons.

This is done using control rods which absorb excess neutrons (reactions require 1 neutron and release 3 so there quickly becomes too many neutrons). They are composed of a material which has many stable isotopes (like Boron) so when the atoms absorb a neutron, they still don't become radioactive. If the reaction is going too fast, the control rods are lowered into the reactor, this increases their surace area to absorb neutrons. If the reaction is going too fast they are rasied up so less neutrons are absorbed.