Random Fact of the Day: In 1974, Hawking predicted that black holes emit small amounts of thermal radiation. This effect has become known as Hawking radiation. If this theory is correct, then black holes are expected to shrink and evaporate over time as they lose mass by the emission of photons and other particles. This effect is proportional to the surface gravity of the black hole. Hence, large black holes emit less radiation than small black holes. Stellar-mass or larger black holes receive more mass from the cosmic microwave background than they emit and thus will grow instead of shrinking. To have a Hawking temperature larger than 2.7 K (and be able to evaporate), a black hole would need a mass less than the Moon. Such a black hole would have a diameter of less than a tenth of a millimeter.