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Question

How does a dunking ducky work? It is a small plastic vessel with a liquid inside. In front of it is a cup with room temperature water. The liquid inside the dunking ducky bubbles up then tips forward then when over the cup it tips back again. It can continue tipping back and forth for a long time.
Asked by: Brooke Piasecki

Answer

This is a great toy! I have one on my desk at school and my students love it! Having it on my desk gives me a great way to teach science without the students knowing it. They simply ask a question; your question. When I answer it they think they are having their question answered but they have really done some very good science; They asked a question!

My 'bird' is made of glass and I suspect yours is too. You could drop it on the floor to find out; that is what happened to mine. Actually, it has happened to several of mine since I have been teaching for a long time. It is important that you know if your bird is made of glass because I do not think it would work with plastic. This whole process works on the transfer of tiny amounts of heat and I do not think plastic can do this as well as glass, very thin glass I might add!

Inside the bird is a substance with a low vaporization point; it evaporates easily. When the bird is just sitting there the temperature inside the head and the body is the same and nothing happens. But, when you get the bird's beak wet everything changes.

Do you know what happens when things get wet? Try this: Put some rubing alcohol on your arm. When you do that part of your arm will feel cool. Why? Because the alcohol is using the heat from your body to vaporize.

This same thing happens on the bird. When the water is on the beak the material from which it is made takes that water and lets it soak the whole head. This spreads the water over a large surface area and allows it to evaporate quickly and easily. When it does the temperature in the head goes down and any vapors from the liquid inside are condensed. What happens? The air pressure inside the head goes down. That means that the air pressure inside the body is now higher and thus the air pressure, which is there because the liquid vaporizes easily, pushes the liquid up the 'neck' into the head. This added weight to the head makes the bird tip over and down he goes into the water.

You can also raise the air pressure in the body by simply holding the bird in your hand. The heat from your hand is more than enough to vaporize the liquid and raise the air pressure in the body. This will push the liquid up into the head very quickly!

When the bird hits the water it soaks up the water and this raises the temperature of the head enough to vaporize the liquid which then raises the air pressure enough to push enough of the liquid back down the 'neck' the bring the bird upright. Then the water on the head is soaked up by the material and it quickly evaporates which causes the vapors in the head to condense and .... well, you get the idea!

This process will go on until enough water has evaporated from the cup into which the bird 'drinks' so that the beak is no longer in contact with the water when it tips over. When that happens in my classroom there is a line of willing children to refill the cup!
Answered by: Tom Young, M.S., Science Teacher, Whitehouse High School, Texas


Science Quote

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Max Planck
(1858-1947)


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