What do elevators, office chairs, and bulldozers have in common? They all work based on the principle of hydraulics.

Hydraulics is the intersection of fluids and mechanics which allows us to use the properties of a liquid to make work easier.

To explain, let’s say you had a water gun, this kind. You pull the handle backward to fill it and push it closed to fire. How does this work? Well, by pushing the handle closed, you would compress the space inside the gun. The water can’t shrink to fit, so it gets forced out through the tiny hole at the front of the gun. This is known as Pascal’s Law: if we put pressure on one side of an incompressible liquid (like water), the pressure is transferred all throughout the liquid and can be used to do work.

In the case of the water gun, the water isn’t doing much more than splashing your friends, but let’s say you have a syringe instead of a water gun.

By pushing down on the plunger, you push the liquid through the tiny needle on the other end, and inject a vaccine into someone’s body. Ta-da! You’ve just used hydraulics!

But why is it necessary? You could just dump a bucket of water on someone’s head, or use an IV for a medical injection.

The reason hydraulics is so common around the world is because of a second property liquids have when compressed: the trade-off of force and speed.

Let’s go back to the water gun. The water comes out from the hole at the end of the gun much faster than you push the handle. How does it do that? Well, when you push the handle, it squeezes a certain volume of water out of the way. The other end of the gun has to get rid of that same volume of water in the same amount of time, but the hole isn’t wide enough for all the water to get out at the same speed as you’re pushing on the handle (and the water can’t be compressed to all fit in the hole because of Pascal’s Law). As a result, the water speeds up so it shoots out of the gun at high speeds, though with much less force.

This switch from high force/low speed to low force/high speed also happens in reverse. A small pipe carrying liquid moving very quickly could move a very heavy object (slowly). In fact, this is how car brakes work.

So there you have it, the basic mechanics that underlie elevators, office chairs, bulldozers, and much, much more.