It’s something almost everyone has seen before — at your grandparents house, in a waiting room, or building suspense in a movie scene — the toy bird that dips down to “drink” from a glass of water over and over again.
But have you ever stopped to think about how it actually works? What keeps it going? Is the water necessary, or just a prop?
You may be surprised to find out that the science behind this little bird is pretty tricky — so much so that it stumped the Grandfather of Gravity himself. Even Albert Einstein reportedly could not figure out what propelled the bird’s seemingly perpetual motion.
Let’s take a quick dive into how this bizarre bird behaves.
Drinking Bird US Patent #2402463 by Miles V. Sullivan, 1945. Photo source: Wikipedia.
Key in understanding how the Drinking Bird works is understanding its construction. Miles V. Sullivan is generally credited with the invention of the toy as we know it today, which he patented in 1945. It should be noted, however, that a very similar “Insatiable Birdie” toy was first seen in China in the early 1900’s.
In his clever design, two chambers make up the head and tail of the bird, connected by a central tube. During the manufacturing process, a liquid is poured down this tube and into the base where it is then frozen so that a vacuum can be applied to remove the air inside before the whole thing is sealed. When the liquid melts, some of it vaporizes, filling the chamber and creating equal pressures in both the head and the base. That’s because this is no ordinary liquid. It is the compound methylene chloride, which has an incredibly low boiling point of 103.3°F.
Now that we know how what’s inside, we can begin to deconstruct what is happening when the bird drinks. To start, the bird must have its beak dunked into a glass of room temperature water (that’s right, the glass of water is essential). By doing this, you are coating the bird’s head, which is covered in a felt material that absorbs the water. The cool water causes the temperature in the head to drop, which causes the pressure in the head to drop as well, and the liquid rises as a result. The liquid rises so much that it goes above the center of gravity and the heavy head tips down for another drink. As the bird tips sideways, the liquid slides down from the head back into the base, equalizing the pressures in both areas and starting the process starts over. This will keep going until all the water on it’s head evaporates.
But why does the pressure in the head drop as the temperature falls? And why does the liquid rise?
The answers have to do with vapor. As the cool water touches the bird’s head, it condenses some of the vapor trapped inside back into liquid, which accumulates at the base of the cylinder. Since there is now less vapor in the head, there is less pressure, which causes the liquid to rise up the cylinder towards that low pressure area.
There’s another invisible force at play here, and it’s one which helps to explain how the bird is powered. When water from the bird’s head evaporates, it is actually giving off heat as it changes states from a liquid to a vapor. Known formally as enthalpy of vaporization, you can feel this effect in action next time you’re stepping out a hot shower. The water on your skin evaporates, taking some of your precious heat with it, leaving you feeling cold. This similar reduction of heat in the bird’s head is what causes the crucial temperature drop that sets everything else into motion.
When you add all of this up, you get what is known as a heat engine. But unlike many of the heat engines that we are familiar with, such as steam-powered trains, the Drinking Bird does not require a fantastic source of heat to operate, such as burning coal. What is incredible about this toy is that its engine runs at room temperature.
You would think for such an incredibly complicated toy with such an elaborate manufacturing process, that this would be a very expensive little gizmo, but you’d be wrong. A little bird told me that The Brain Shoppe has them for just $9.99, and that’s definitely worth two in the bush!