What it shows:
Three containers are filled with water to the same depth, and each has the same base surface area (see figure 1). Since the pressure and area are the same in each container, the force should be the same (pressure = force/area). So how come the scales read different values?
How it works:
It is true that the pressure at the bottom of the water inside each container is the same, but that's not what the scales are measuring. The scales measure the weight of the containers and water on top of them, and that is different for each.
figure 1. equal pressures, but equal weights?
Setting it up:
The containers are made of 5mm Plexiglass. Each has a base 16cm of side, and a height of 15cm. They each sit on platform scales 1 To make the water more visible, a dash of food coloring brightens things up.
This paradox frequently appears as a text book question on hydrostatics; it's much nicer to see that it really works.
1 Metro Equipment kitchen scales, as described in the Loaded Beam demo