Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to the passage of an electric current. The reaction has a standard potential of −1.23 V.
That means if you supply a potential difference greater than 1.23 volts you can split water! Say with a 9V battery, you can make some gas!!!
NOTE: The Hydrogen side makes twice as much gas as the oxygen side because it is H2 O.
Double NOTE: Water Electrolysis is how the International Space Station gets is oxygen. So you are doing science in the same league (Lego league?) as NASA science!
Use simple office supplies and Legos (optional) to separate the hydrogen from the oxygen in water.
30 minutes with Legos
It is just fun to just make Hydrogen at home.
Using NaCl (table salt) in as an electrolyte solution results in chlorine gas rather than oxygen due to a competing half-reaction (Wikipedia: Water Electrolysis).