A neutrino is one of the smallest particles we know of — far smaller than an atom, with no electrical charge and almost no mass. What makes it famous is that it hardly touches anything. Neutrinos pour out of the sun in unimaginable numbers and pass straight through the earth, through buildings, through your body, day and night, without you feeling a thing; roughly a hundred trillion go through you every second. Because they pass through everything, they are extremely hard to catch. Physicists build enormous detectors — tanks of water or ice, buried deep underground or sunk into the Antarctic — and wait, sometimes for months, for a handful of neutrinos to happen to hit something. Two more things worth knowing. Neutrinos come in three types (physicists say "flavours") and, surprisingly, change from one type into another as they travel — a discovery that won a Nobel Prize in 2015 and proved they have some mass after all, which the original theory said they should not. And because they escape from places light cannot leave, such as the middle of an exploding star, they let us see events we could never otherwise watch. The plural is neutrinos. This is a physics word, but it appears often in popular science writing and news, and among students of science it is used with a certain affection.