Transponder (aeronautics)

A transponder in a plane is like a special radio that can send and receive signals. It is a little box that sits inside the plane and talks to other things outside the plane.

One way it talks is by sending a special code to the control tower when the pilot turns it on. This code tells the control tower which plane is talking to them. It is like when you pick up the phone and say "Hi Grandma, it's me, Susie!"

Another way it talks is by listening to signals from other planes or radar stations. When it hears a signal, it sends a signal back so the other plane or radar station knows it's there. It is like when you wave to someone and they wave back to you.

The transponder helps the pilot and the control tower communicate and know where all the planes are in the sky, so they can avoid bumping into each other. It is like playing a game of tag with your friends, but you always know where they are and where you need to go to avoid tagging each other.