Drinking Duck

The toy duck that has a drinking problem.  As long as a temperature gradient exists between the top and bottom bulbs the bird continues to appear to take sips from a glass of water.  This is driven by the endothermic heat of vaporization.


  • the drinking duck toy
  • a glass of water (for effect)


  1. Duck is placed on the table, and then it drinks.


The duck is a two chamber structure.  The top chamber is covered with a fuzzy material that soaks up water when the beak dips into the cup in front of it.  Then the water on the fuzzy material evaporates. Since the heat of vaporization is endothermic, this cools the head chamber.  The inside of the bird has a volatile solvent in it so there is some significant amount of vapor phase molecules.  The vapor-phase of the filling liquid contracts due to cooling and roughly according to the ideal gas law.  The contracting gas lowers the pressure in the head bulb, drawing the liquid from the bottom chamber through the tube and up to the top chamber.  This makes it top heavy so that it tips over and "takes a drink", re-wetting the felt covering the head.  When it tips over, a bubble of the warmer vapor from the bottom layer displaces the cooler vapor in the top and the liquid drops back to the lower bulb.  The bird is reset and tips back up to start the cycle over.  The process will continue as long as the fuzzy material on the head does not dry out.  This is not a perpetual motion machine: Energy is being lost from the top bulb material when it is transferred into the molecules that are vaporized from the fuzzy head material.  That energy is carried away as kinetic energy in the evaporated molecules.  The energy is replenished from the warmer lower bulb chamber.  As long as there is a temperature difference between the upper and lower bulbs, this gradient drives the process.  If the bottom bulb could not be maintained warmer than the top bulb, the duck would stop drinking.


Special Safety Notes

  • No safety hazards
The Drinking Duck.
Diagram showing the temperature equilibration that resets the duck.