How do putt putt boats move around?

When water rushes up the pipes into the hot boiler, it instantly vaporises. This causes it to expand and push the water back out of the pipes. According to Newtons third law, there must be an equal and opposite reaction to imparting all that momentum to the water and that is to push the boat in the opposite direction, much like rocket propulsion.

The next stage of the cycle involves the steam in the boiler condensing again, contracting and allowing water to rush up the pipes to start the process again.

This page explains it much better than I can, http://www.sciencetoymaker.org/boat/howBoatWorksl.html