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Drinking birds, also known as dippy birds and dipping birds,[1] are toy heat engines that mimic the motions of a bird drinking from a fountain or other water source. It is sometimes incorrectly considered a perpetual motion device.

Construction and materials
A drinking bird consists of two glass bulbs joined by a glass tube (the bird's neck). The tube extends nearly all the way into the bottom bulb, and attaches to the top bulb but does not extend into it. The space inside the bird contains a fluid, usually colored. The fluid is typically dichloromethane, also known as methylene chloride.

Air is removed from the apparatus during manufacture, so the space inside the body is filled by vapor evaporated from the fluid. The upper bulb has a "beak" attached which, along with the head, is covered in a felt-like material. The bird is typically decorated with paper eyes, a plastic top hat, and one or more tail feathers. The whole setup pivots on an adjustable crosspiece attached to the neck.

Despite the drinking bird's appearance and classification as a toy, some safety considerations apply. Early models were often filled with highly flammable substances. The fluid in later versions is nonflammable. Dichloromethane can irritate the skin on contact, and the lungs if inhaled; it is a mutagen and teratogen, and potentially a carcinogen. The intact toy is leakproof and completely safe, but if broken hazardous dichloromethane is released. Dichloromethane evaporates quickly; good ventilation after a spill will dilute and disperse the vapor.

Physical and chemical principles
The drinking bird is an interesting exhibition of several physical laws and is therefore a staple of basic chemistry and physics education. These include:

The combined gas law, which establishes a proportional relationship between temperature and pressure exerted by a gas in a constant volume.
The ideal gas law, which establishes a proportional relationship between number of gas particles and pressure in a constant volume.
The Maxwell–Boltzmann distribution, which establishes that molecules in a given space at a given temperature vary in energy level, and therefore can exist in multiple phases (solid/liquid/gas) at a single temperature.
Heat of vaporization (or condensation), which establishes that substances absorb (or give off) heat when changing state at a constant temperature.
Torque and center of mass
Capillary action of the wicking felt.

How it works
The drinking bird is a heat engine that exploits a temperature differential to convert heat energy to a pressure differential within the device, and perform mechanical work. Like all heat engines, the drinking bird works through a thermodynamic cycle. The initial state of the system is a bird with a wet head oriented vertically with an initial oscillation on its pivot.

The process operates as follows[2]:

The water evaporates from the felt on the head.
Evaporation lowers the temperature of the glass head (heat of vaporization).
The temperature's drop causes some of the dichloromethane vapor in the head to condense.
The lower temperature and condensation together cause the pressure to drop in the head (ideal gas law).
The pressure differential between the head and base causes the liquid to be pushed up from the base.
As liquid flows into the head, the bird becomes top heavy and tips over during its oscillations.
When the bird tips over, the bottom end of the neck tube rises above the surface of the liquid.
A bubble of vapor rises up the tube through this gap, displacing liquid as it goes.
Liquid flows back to the bottom bulb (the toy is so designed that when it has tipped over the neck's tilt allows this), and vapor pressure equalizes between the top and bottom bulbs
The weight of the liquid in the bottom bulb restores the bird to its vertical position
The liquid in the bottom bulb is heated by ambient air, which is at a temperature slightly higher than the temperature of the bird's head.
If a glass of water is placed so that the beak dips into it on its descent, the bird will continue to absorb water and the cycle will continue as long as there is enough water in the glass to keep the head wet. However, the bird will continue to dip even without a source of water, as long as the head is wet, or as long as a temperature differential is maintained between the head and body. This differential can be generated without evaporative cooling in the head—for instance, a heat source directed at the bottom bulb will create a pressure differential between top and bottom that will drive the engine. The ultimate source of energy is the temperature gradient between the toy and the surrounding environment—the toy is not a perpetual motion machine.