Fig. 1  The funny "drinking bird". Real Video clip [Low Res | High Res]

Fig. 2  The structure of the "drinking bird".

Fig. 3  The "drinking bird" drinks.

Without any external energy provided, how can this little toy bird keep repeating its action of drinking water?

"The drinking bird" can be regarded as a classical scientific toy (Fig. 1). It requires neither electricity nor winding, but it can keep dipping into the water repeatedly. Actually the operation of this little toy involves the physical principles of capillarity effect, latent heat of vaporization and vapour pressure, and it is worth introducing here.

The body of the drinking bird is actually a long glass tube that connects to the flasks at the head and the bottom (Fig. 2). It has a very narrow metal mouth, inside which there is a cotton string connected with the cloth that is wrapped around the head. Some volatile liquid is placed in the bottom flask, so that the glass tube is filled up with vapour. The glass tube extends to the bottom flask, with the liquid soaking over the tip of the tube. The body of the bird is supported by a pivot so that it can freely rotate.

When the bird stands straight, the water that it absorbed earlier evaporates at the cloth on its head, carrying away the latent heat of vaporization. This cools down the vapour inside the head flask. The vapour pressure in the glass tube is reduced and thus the pressure at the bottom flask pushes the liquid into the glass tube. Therefore the standing bird loses its balance and lean forward to dip into the water on the little plate with its metal mouth, completing the "drinking" action.

How does the bird "drink"? After it "drank", how can it resume its straight standing position? Due to capillarity effect, when its mouth dips into the water, water is absorbed by the cotton string and reaches the cloth that is wrapped around its head. When the bird "drinks", its body is at the horizontal position (Fig. 3). The spaces inside the bottom flask and the head are united, and the unbalanced vapour pressure vanishes. The liquid flows back to the bottom and thus the bird stands straight again, and the whole action is repeated.

After reading the above paragraphs thoroughly, one would notice that many physical principles are involved in such a short description. Can you list them all?