When smoke flows out of a chimney, it moves through a narrow opening called a flue. Before you light a fire in the fireplace, make sure the flue is open!
Don't mistake flue for its homonym, flu — rather than an illness, a flue is a duct or pipe that's connected to a stove, heater, furnace, or fireplace. Smoke or gas can flow freely through the flue, which allows the appliance to work properly and vents any nasty exhaust away from people. Experts disagree about the origin of this word, but many trace it back to a Germanic root meaning "to flow."