Pascal's Law says that when pressure that is placed on liquid in a container, it will disperse equally to all parts of the container. It was discovered by the French mathematician and philosopher Blaise Pascal in the 1600s. He was the inventor of the hydraulic press, the most common application of the Law.

An easy demonstration of Pascal's Law is to step on a balloon. When pressure is placed on the top of the balloon, the air within is dispersed to the rest of the balloon resulting in either a misshapen balloon or one that distorts to the point it can, and then breaks under the pressure. This particular example is helpful for visualization, although Pascal's Law technically applies to a container filled with liquid, not air.

To transfer the experiment to water, a common demonstration uses a bottle, a balloon, and some matchstick heads. Fill the bottle with water nearly to the top and drop the matchstick heads into the water. When at rest, the matchstick heads will float at the top of the water. Stretch the neck of the inflated balloon over the bottle, and press a finger onto the balloon, going into the bottle. The matchstick heads will float down into the bottle as the pressure increases on the water.

Pascal's Law also explains why a wine bottle may crack or break when someone tries to replace a cork that has been removed. When bottled, the air had initially been sucked out and replaced by a cork. When the cork was popped, air entered the bottle. To replace the cork means to compress the new air onto the wine in the bottle. The wine can only compress so far before the liquid must press outward over the entire surface of the bottle, thereby cracking the bottle.

The most common use of Pascal's Law is in a hydraulic system. In a hydraulic press, a liquid that can not be compressed is placed in a small piston. Pressure is placed on the liquid by a handle pressed by the user. The liquid is transferred to a larger piston which then lifts. The pressure used to press the smaller piston down is displaced evenly to the whole sealed unit with less effort than would be required to lift the larger piston in its own, much like a lever lifting a rock. This concept is used in hydraulic mechanisms of many types, such as the braking systems on aircraft, and on some vehicles as well.