The drinking duck


The duck is composed of two spherical cavities joined by a tube which starts in the neck of the puppet and goes on inside the lower cavity quite until the bottom. The object can rotate on an horizontal axis set about in the middle of the cavities. The lower cavity contains a very volatile liquid with a tension of steam which is strongly correlated with temperature; the higher cavity, the "head" of the duck, is coated by felt that can absorb water.
The duck is going to drink..
A foreground of the duck

If the head of the duck is dry, all the colored liquid remain in the lower cavity: center of mass of the system is in this case under the axis of rotation. The result is that the weight produces a couple that trends to bring the duck back in the vertical position of stable equilibrium.

If you wet the felt that coats the head, you observe that the colored liquid starts rising on the neck. Because of this rising, the center of mass of the duck goes up, until it is over the axis of rotation. At this point the twisting force due to the weight makes the duck incline to the bottom.

Finally, when the beak beats the glass, the tube is so inclined thus it doesn't draw in the lower cavity. Consequentially the liquid immediately goes down and the duck returns to the vertical position.

We want also to understand why suddenly the liquid starts rising. The explanation is very simple: due to the evaporation of the water, that wets the felt, the temperature of the head decrease. Then the head reaches a lower pressure than the cavity at the bottom and so the higher pressure in the cavity pushes up the liquid.

A curiosity: : we don't need the water in the glass (it is only a "special effect"), in fact it is enough to wet head. Before the head is completely dry, a lot of time has passed and we can observe the duck going up and down many times.