A drinking duck

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.