Stupid Question ™
June 1, 2000
By John Ruch
Q: There’s a toy shaped like a little bird that dunks its head repeatedly in a glass of water until the water runs out. How does it work?
A: You mean it actually works? I’ve had two of these things on my desk for five days and they still won’t dip.
The item is known as a Drinking Bird, a Dippy Bird or, in my case, the Non-Flammable Drinking Happy Bird. (The box helpfully adds the following about my non-flammable bird: “Flammable. Keep away from flame.”)
A physics experiment very badly disguised as a toy, it consists of a glass tube with bulbs at either end and a metal bar clamped across the middle to hold the tube vertically. The top bulb is covered with a red felt-like material, and has a Styrofoam “beak” covered with the same fabric. This “head” also has stick-on eyes and a blue plastic top hat.
The bottom bulb is bare except for a green feather glued on to complete the avian illusion (or allusion, really). Inside you can see a blue or red fluid—usually the paint remover methylene chloride.
The metal bar across the middle balances the bird on two white plastic bird legs that end in red boots. The bar is loose to allow the bird to rock backward and forward, and slightly bent so that it tends to rock forward.
If you get the head wet, the fluid inside rises and the bird dips forward. Then the fluid goes down and the bird goes upright again. Put a cup of water in front of the bird for it to dip its head in, and it will keep dipping until the water is gone (and even for a while afterward). That can be a long, long time if you keep refilling the cup.
A brand new bird has the same temperature and pressure inside both bulbs. But when the head gets wet, it cools down, and keeps cooling as water evaporates from the felt.
The cooling lowers the pressure in the top bulb (by condensing the volatile fluid’s vapor), which increases the pressure in the bottom bulb. This forces the fluid up the tube. When it rises above the metal bar, it unbalances the bird, which then dips forward.
All of this has lowered the pressure in the bottom bulb, causing some of the highly volatile fluid to evaporate. This vapor then enters the tube and starts floating upward toward the top bulb. It raises pressure in the top bulb, causing the fluid to run back down into the bottom bulb. The center of gravity shifts in the process, and the bird goes vertical again.
If the head is kept wet, it will cool again and start the whole process over.
But water isn’t essential—the key is simply keeping the top bulb cooler than the lower bulb. Paint the bottom bulb black and put it in strong sunlight so it’ll heat up, and the bird will dip without any water at all.
On the other hand, wetting the bird’s head does you no good when the atmosphere is too humid for rapid evaporation to occur. The heat transfer won’t happen quickly enough, and your sopping bird will twitch slightly and then just sit there.
Which is why I shouldn’t have been playing with my Drinking Birds during Columbus, Ohio’s infamously rainy Memorial golf tournament.