The only really good way to slow down a speeding neutron is to have it collide with something its own size. (The same is true with a pinball: if it runs head on into a stationary pinball, it will come to a dead stop. If the collision is not quite head on, it will slow down dramatically.) There's only one atom the same size as a neutron: hydrogen, the lightest of all the elements. If the Moon's crust contains lots of hydrogen at a certain location--say, a crater with water in it--any neutron that bounces around in the crust before heading out to space will cool off rapidly. When Lunar Prospector flies over that crater, the Neutron Spectrometer will see a surge in the number of cool (or "thermal") neutrons, and a dropoff in the number of warm ("epithermal") neutrons.">