A simple salt-water battery can light a low-power LED under the right conditions, especially when several cells are connected in series. The experiment is useful because every part has a clear job.
Electrolyte versus fuel
The salt water is mainly an electrolyte. Dissolved ions allow charge to move through the liquid. The useful electrical energy comes from oxidation and reduction reactions involving the electrode materials. One electrode is gradually consumed or chemically changed. That is why the cell eventually weakens.
Why two different materials matter
Different electrode materials have different tendencies to participate in electrochemical reactions. Pairing appropriate materials creates a voltage difference. The exact voltage and current depend on chemistry, surface area, concentration, temperature, spacing and internal resistance.
What to measure
Record open-circuit voltage, loaded voltage, current through a known resistor and how those values change over time. Weigh electrodes before and after longer tests if possible. Compare identical cell sizes with different electrode combinations. This turns a fun demonstration into actual data.
The important limit
A salt-water cell is not a perpetual source of energy. If an electrode corrodes or reacts, chemical free energy is being converted into electrical energy. A better long-term design must address electrode lifetime, reversibility, cost and safe recharging.