Resistojet rocket

A resistojet can be thought of as a "space heater attached to a water hose". Of course, it's much more sophisticated than that! But it is a good analogy, because it emphasizes the two characteristic parts of the system: A source of heat produced by electricity through some sort of a resistor, and a source of (typically non-reactive) fluid.

Resistojets have been flown in space, and do well in situations where energy is much more plentiful than mass, and where propulsion efficiency needs to be reasonably high but low-thrust is acceptable.