A few transparent tubes are partially stuck into a water-basin (the water is coloured by strawberry sirup). The water-level in each of the tubes is equal. This is what one would expect. Now air is blown over the upper end of the tubes. The air is led through a cone-shaped pipe, which is connected with the tubes. Observe how the water-levels in the tubes become different.
Explanation 1: The air is flowing fast in the narrow region of the pipe. There the water-level in the tubes is high, meaning that the pressure of the air is low. From this experiment we can learn that the pressure of a gas is low where the speed of the gas is high.
Explanation 2: The law of the 'communicating vessels' dictates that de water-level in each tube should be the same, as initially is the case. An important condition for the validity of this law is, that the pressure (of the air) on all water-surfaces must the same. When the air is blown, the conclusion must be that the pressure of the air in the tubes is not equal. Where the pipe is narrowest, the water-level is highest, meaning that the pressure of the air is lowest. In this area of the pipe the air is flowing fastest. This principle has found many applications, for instance, the wing of an airplane (providing the lift through pressure differences over and under the wing) and the carburettor of an engine.