Pascal's principle : Pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid, as well as to the walls of the container.
A common application of this is a hydraulic lift used to raise a car off the ground so it can be repaired at a garage. A small force applied to a small-area piston is transformed to a large force at a large-area piston. If a car sits on top of the large piston, it can be lifted by applying a relatively small force to the smaller piston, the ratio of the forces being equal to the ratio of the areas of the pistons.
Are you getting something for nothing here? Absolutely not. The hydraulic lift is very similar to a lever, where a small force applied through a large distance can move a heavy object a small distance. The work required to lift the heavy object equals the work done by the small force. A lever uses a bar and a fulcrum to transform the work - the fluid does that in the hydraulic lift.