A duct is a tube or channel that carries air, liquids, or other substances from one place to another. In buildings, metal ducts hidden in walls and ceilings transport heated or cooled air from the furnace or air conditioner to every room. When you feel air blowing from a vent in your classroom or bedroom, it traveled through a system of ducts to reach you.

Your body has ducts too. Tear ducts carry tears from glands near your eyes down into your nose, which is why your nose runs when you cry. Other ducts transport saliva to your mouth and bile to your digestive system.

Engineers design duct systems carefully to make sure air flows efficiently through buildings. If ducts leak or get blocked, rooms won't heat or cool properly, which is why maintenance workers may need to clean or repair them.