A tube is a hollow cylinder that things can move through or be stored in. Picture a garden hose: water flows through its circular opening from one end to the other. That's a tube. Toothpaste comes in a tube that you squeeze to push the paste out. Mail used to be sent through pneumatic tubes that whooshed packages through buildings using air pressure, and some banks still use these systems at drive-through windows.

The London Underground is nicknamed “the Tube” because its round tunnels look like giant tubes burrowed beneath the city. In your body, different tubes do different jobs: your esophagus is the tube that carries food to your stomach, your trachea is the tube that brings air to your lungs, and blood vessels are tubes that carry blood throughout your body.

The word can also mean an old-fashioned television, since early TVs used cathode ray tubes to display pictures. When your grandparents talk about watching “the tube,” they mean watching TV.