The same electrodes can behave very differently depending on what surrounds them. Here, copper electrodes sit in two beakers — one filled with distilled water, one with sea water. Electricity barely moves through the distilled water. In sea water, though, the current jumps high enough to light a bulb. What sits between the electrodes matters as much as the electrodes themselves.

Distilled water and sea water look nearly identical, but they behave very differently when current tries to pass through them. You set up two beakers with copper electrodes — one filled with distilled water, one with sea water — and connect each to a battery, a light bulb, and an ammeter. The voltage stays nearly the same in both beakers. The current, however, jumps from almost zero in distilled water to a level that lights the bulb in sea water. That difference comes from dissolved salt ions, which carry charge through the liquid in a way that pure water simply cannot.

For current to flow, charged bits need a path through the material. You set up two beakers with copper electrodes — one filled with distilled water, one with sea water — then complete each circuit with a battery, a light bulb, and an ammeter. The voltage stays nearly the same in both beakers. The current, however, jumps from almost zero in distilled water to a level that lights the bulb in sea water. Dissolved salt ions give sea water its conducting power; distilled water has none of them, so charged bits have no way through.

For a circuit to run something, it needs a continuous loop that lets power flow from the source, through a wire, and back. Here, a battery, a light bulb, and an ammeter complete that loop through two beakers — one filled with distilled water, one with sea water. The voltage stays nearly the same in both. The current, however, jumps from almost zero in distilled water to a level that lights the bulb in sea water. That difference comes from dissolved salt ions carrying charge through the liquid, completing the loop in a way that pure water cannot.

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.