"s were among the most paradoxical members of the zoo of elementary particles that were discovered after the war. Produced during radioactive decay, they supposedly had neither charge nor mass and they traveled, consequently, at the speed of light. Their only interaction with the world (besides gravity) was by something called the "weak" force, which causes some kinds of radioactive decay. It was so weak that, according to calculations, a typical neutrino could pass through a million miles of water unhindered—stars and planets were transparent to them."