The duck, whose head is wet, leans forward and tries to drink water from the glass. When the duck touches the rim of the glass it moves back to the vertical position and then again leans forward.

The red hat covers one of the two glass bulbs joined by a small tube (the bird's neck). There is a liquid inside that evaporates easily. The level of the liquid in the tube slowly rises till the bird leans forward. Then the liquid flows down to the lower bulb.

The movement of the liquid is the result of the pressure difference in the bottom and top bulbs. While the water evaporates from the wet hat, the temperature of the hat falls and, subsequently, the pressure in the top bulb decreases. The pressure difference sucks the liquid from the bottom bulb and the bird's centre of gravity moves up until the bird leans forward. While the bird is leaning, the pressures become equal and the liquid flows down. It seem to be example of perpetual motion.

The duck moves faster if it dips its head in alcohol. You start nodding off when you are tipsy...

Three processes taking place in the drinking duck toy can be introduced one by one:

1) water evaporation lowers T₁ temperature which in turn lowers the pressure p₁ in the upper bulb (like in a psychrometer).

2) Reaching pressure balance between the upper and lower bulbs brings about the rise of the level of liquid filling the lower bulb (picture 1) which in turn moves the duck's centre of gravity upwards. As a result of it the duck leans forward gradually until the centre of gravity reaches the suspension point.

3)Once the centre of gravity reaches the level above the suspension point, the duck tilts its head rapidly forward, touching the rim of the glass (picture 2). At the same time it moistens its beak with water from the glass and, as a result of leaning towards the glass, opens up a passage through which vapour circulates between two bulbs. This in turn redresses the balance in temperature and pressure between the bulbs and causes the liquid flow to the bottom bulb. As a result the centre of gravity lowers and the duck assumes its standing up straight posture. And the whole cycle begins to repeat.

This is quite a small thermal engine.