Water cools the neutrons which makes them slower, they only move with 2,2 km per second after having cooled down. On their way they are looking for one uranium atom so that the nuclear fission can go on and on.
The moment there are too many neutrons in the nuclear reactor the control rod sucks them up.
These control rods are also used to turn off the nuclear power station. In fact the control rods are one main part in the reactor which is terribly important. Imagine they function like an enormous brake!
Doesn't that sound great? Actually it is, but the negative side is that the family uranium 235 produces horribly high amounts of radio activity after having exploded.
