This is excellent !
A little tin boat that has a U shaped tube with both ends at the back, just under the water line. At the bend in the U is a slightly larger chamber that has a small candle or burner under it.
It probably easier to point you to a web site to see how it works (The Pop-pop Boat Page), I just love the idea that it will happily power itself up and down the bath - but it has no moving parts !
