In Poland, on Easter Monday, people use to flush friends with cold water. They say, it brings health and luck. Now You can buy many ready pistols for this.

This coloured pistol fulfils two different functions thanks to two containers it has. One container is placed at the top and is filled with water and little colourful rings. There are also two buttons. If you press one of them a piston moves and pushes the liquid away. As a result, the coloured rings float.

If you pull the trigger, the piston moves and exerts pressure on water which has been sucked to the piston cylinder earlier. At the other end, there is a long and narrow tube through which water is shot outside. This narrow tube makes it possible for the water to reach a distance of 5 metres from the height of 1 metre. The narrower the tube is, the higher the speed of water.

The surface of the piston is comparatively small and it is enough to press it slightly to increase the pressure in the container considerably. Huge hydraulic presses work on the same principle which is known as Pascal's Law.

The amount of water being pushed by the piston inside at a given moment is equal to the amount of water shooting in a stream outside. This is what the law of conservation of matter (mass) postulates. The continuity equation is the conclusion inferred from the law in question. The product of the density of water ρ, the surface of the piston S₁ nd the velocity v₁ of the water flow in a chamber with a piston (which stands for the mass of water flowing in a unit of time) has to be equal to the product of the density of water, the surface of the muzzle S₂ and the velocity in the muzzle v₂:

ρS₁v₁ = ρS₂v₂ .

As illustrated by the equation, the velocity of a jet of water is proportional to the velocity of the piston movement and it is greater by a ratio of the surface of the piston by the surface of the muzzle.

The hydraulic press operates on a similar principle (see the picture). If force F₁ is applied to a piston with S₁, surface, the force applied to the other piston will be greater by a ratio of the surface of the pistons.

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You can construct your own pneumatic press using two syringes of different diameter joined with a piece of plastic hose. Fill them with water! Air can be compressed quite easily and the press would not operate properly.