this is a small toy driven by evaporation, which is commonly called "Drinking bird". Looks extremely cute:

(eyes look like mentally retarded …... )

if you put it close to a glass of water of the right height, and then wet the beak in the water first, and then you can see it start to wobble and drink, the scene is very happy:

here is a video:

so, how do you do this?

first of all, let's take a look at the structure of "drinking waterbirds". This thing is actually a sealed glass container on a shelf. The structure is as follows:

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Bird's head and Bird's belly are connected by a glass tube, which contains some volatile liquids (such as dichloromethane, boiling point 39.6 °C). The lower end of the glass tube is immersed in the liquid. The "beak" connected to the "bird head" is covered with a felt material that can absorb water.

the air is removed when the device is manufactured, so the part of the cavity is filled with volatile steam from the liquid. The cavities in the belly and head of the bird are filled with steam. Note that the two parts of the gas are separated by liquid. At first, the steam pressure of the two parts is balanced.

next, you need to wet the beak manually, and then the water absorbed in the felt material on the beak evaporates, causing the bird's head to cool down.

under thermal imaging, you can see the temperature drop in the bird's head:

the cooling of the head leads to a decrease in the steam pressure in the upper glass cavity (on the one hand, part of the steam condenses. On the other hand, the constant volume of the temperature drop itself will lower the air pressure, thus breaking the original balance. The air pressure in the lower cavity is higher, and the liquid below is pressed into the glass tube.

after the liquid rises, the bird becomes unstable and falls forward. After tilting, the lower end of the glass tube runs above the liquid level, causing the liquid in the tube to flow back down, and the upper and lower air pressure becomes balanced again. When the liquid returns to the lower end, it will drive the bird to stand upright.

and if the bird puts water in front, its beak will be soaked again when it falls forward, and the above cycle will continue to occur. In this way, "drinking waterbirds" can continue to drink water.

this small toy can help people understand some basic physical concepts, but I think its biggest advantage is that it is very magic, and the video looks especially funny.

it is worth noting that the drinking waterbird is not a perpetual motion machine, it must constantly obtain water for evaporation from the outside in order to maintain its movement.

Thermal imaging and schematic drawings are from: Sixty Symbols