Hydraulic scale

HOW

1. Stand in the middle of the scale.
2. Observe the liquid level in the tube. Check the level the liquid reaches. It roughly corresponds to your weight in kilograms.

WHY

A tube of dyed water connects to the platform you are standing on. When you push on it with your body weight, you increase the pressure inside the tube. With a set liquid, the pressure, known as hydrostatic, depends only on the height to which it rises in the tube. More mass means more pressure on the platform. This translates into a higher pressure in the tube and therefore a higher level of liquid to balance this pressure.