An air conditioner is a machine that cools the air inside a building and removes humidity to make hot weather more comfortable. When summer temperatures climb into the 90s or higher, an air conditioner pulls warm air from inside, cools it down, and pushes the refreshed air back into the room.

Air conditioners work by using a special liquid called refrigerant that absorbs heat from indoor air and releases it outside. That's why the back of an air conditioner (or the outdoor unit for central air conditioning) always feels hot: it's dumping the heat it removed from inside your house.

Before air conditioning became common in homes in the 1950s and 1960s, people relied on fans, open windows, shade, and architectural tricks like high ceilings and wide porches to stay cool. The invention of air conditioning transformed where people could comfortably live and work, helping cities in hot climates like Phoenix, Houston, and Miami grow dramatically.

You'll also hear people use the shortened term AC or say they're running the air when they turn on the air conditioner. Some people keep their homes very cold with AC, while others use it sparingly to save electricity or because they find the artificial coolness uncomfortable.