An ion is an atom that has gained or lost a tiny charged part, giving it the ability to carry electricity through a solution. When salt dissolves in water, it breaks into ions that move freely through the liquid, carrying charge as they go. Distilled water has no such particles — so when you connect copper electrodes to a battery in distilled water, the current reads almost zero and the bulb stays dark. Switch to sea water, and the dissolved salt ions carry charge across the circuit, pushing the current to a level that lights the bulb.