Xenon is one of the chemical elements — a gas with no colour and no smell. There is a tiny amount of it in the air we breathe, so small that you could never notice it. Its symbol is Xe. It belongs to the group chemists call the noble gases, which means it almost never joins with other things; it just stays itself. Where students actually meet the word is in lights: put xenon in a lamp and send electricity through it, and it gives a very bright white-blue light. Car headlights that look brilliantly white instead of yellow are often called xenon headlights (also called HID lights), and the flash in older cameras and in lighthouses used xenon too. So the word lives in two places — the chemistry textbook, and the shop where people talk about car lamps. It is a noun and you do not put "a" in front of it: say "xenon" or "xenon gas", not "a xenon". The first sound is /z/, not /ks/ — "ZEE-non".