Hydraulics
Hydraulics is using liquid pushed through tubes to move or lift heavy things.
Think of it this way
When you push down on a full plastic bag, the liquid inside pushes outward in all directions. That outward push can lift objects or press against walls of a container. A sealed bag with water under a heavy block shows this — press one side down, and the other side rises. The liquid carries force from one spot to another.
Explaining hydraulics by grade level
Push water in a tube. It pushes out the other end. The water moves things for you. That is how hydraulics work.