A cage is a structure that keeps an animal captive. If you decide to raise a tiger in your bedroom, it's best to have a sturdy cage.
Cage is equally at home as a noun or verb. For instance, a batting cage is fun, but a free bird does not desire you to cage it. A cage of an elevator is great to ride to the 30th floor, but not so great if you’re riding it down into the mines and it breaks 40 feet below the earth.