Getting a Mogwai/Gremlin wet causes them to multiply, but where does the energy for this come from? Are the Mogwai using the water itself as energy to fuel their split? Is the water consumed? Or are the Mogwai breaking the rules of thermodynamics and could be used to create free energy?
Although I can't find a screenshot of it at the moment, there was a scene from the first movie where the Gremlin jumps in the pool. Smoke starts billowing out, flashes of light are seen underneath...
Based on that, I'd guess some sort of nuclear fission. In that case, the "smoke" would actually have been steam, while the flashes of light could be some sort of Cherenkov radiation.