The "putt-putt" or "pop-pop" boat is a classic toy rarely seen in the 21st century, and a fascinating example of a heat engine at work. A small heater using vegetable oil boils water, forcing out water and steam from the tailpipe of the boat. After the steam and water are expelled water is pulled back into the boiler. This process produces a net force in the forward direction, propelling the boat forward. Refer to the geometry of the boat boiler system (above) to further understand this phenomenon.
This is similar to the processes by which the inverse sprinkler is driven in the forward and the reverse directions to cause the direction of rotation to reverse for the inverse sprinkler.
Note: Be sure to completely fill the boiler chamber with water before heating it. Practice lighting the wick.