A warmer is a thing — very often a small machine or gadget — whose job is to make something warm or to keep it warm. It nearly always appears with a word in front saying what it warms: a hand warmer you hold in your pocket in winter, a bottle warmer for a baby's milk, a food warmer or plate warmer in a restaurant that keeps dishes hot before serving, a leg warmer that dancers pull over their calves. On its own, plain 'warmer' is rare; it is the second half of a compound almost every time you meet it. English teachers borrow the word too: a warmer is a short, easy activity at the start of a lesson to get the class talking — the classroom version of a warm-up. And of course 'warmer' is also just the comparative of warm ('It's warmer today than yesterday', 'Come and sit nearer the fire, it's warmer here'), and in the children's hiding game, 'Warmer! Warmer!' means you are getting closer to the thing you are looking for. Same spelling, and you tell them apart by whether the word is naming a thing or comparing.