Saltwater Conduction

Copper plates are secured to the ends of a fish tank. A cable connects one plate to a flashing blue light and a siren. The other plate is connected to a voltage source. When a large amount of salt is poured into the water, the blue light begins to flash and the siren wails.

How does it work?
Normal tap water conducts very poorly, while distilled water is non-conductive. Dissolving table salt in water forms positive sodium ions and negative chlorine ions. Each chlorine atom has one electron that is taken from a sodium atom, and therefore exists in the water as a charged ion. These ions are free to move in the water, similar to the way electrons can move in a cable. The water can therefore conduct electricity, allowing the blue light and siren to do their thing.