Solenoids

A solenoid is an inductor wound on a hollow core, which allows an iron rod (plunger) to be attracted toward the center. Usually there is a method (other than a spring) to push/move the plunger away from the center when the solenoid is not energized. Used in door un-latching, mechanical counters, relays and other things that need to be moved in a line.

This answer could prompt the questions:

What is an inductor? What is iron? What is wound?

And anything else he can think of rather than reading and understanding the copious information already on the web and in text books.

Philip