Hydraulic telegraph

Imagine you have two cups, one at the top of a hill and one at the bottom of the hill. You want to tell your friend at the bottom of the hill something important, but you can't shout loud enough for them to hear you.

So you decide to use a hydraulic system, which is like a fancy way of using water to send messages. You fill the cup at the top of the hill with water and put a tube in it. When you push down on the water in the cup, it goes through the tube and squirts out into the cup at the bottom of the hill.

Your friend can then see the water coming out of the cup and know that you're sending a message. They can also count how many times the water squirts out to understand what you're saying. For example, one squirt might mean "yes," and two squirts might mean "no."

This is how a hydraulic telegraph works. Instead of cups, tubes and water, it uses a system of tubes and water to send messages over long distances. It was used thousands of years ago before phones or computers were invented, and was a very important way for people to communicate with each other over long distances.