Here is a very fun and easy activity using pencils to separate hydrogen and oxygen from water.

You will need:

Water, 9V battery, two pencils sharpened on all sides, electrical wire, salt

Now try this:

1. Connect one electrical wire to the positive side of the battery and connect another electrical wire negative side of the battery. 
2. Now connect the other ends of the wire to the graphite (the "lead" of the pencil) on the two pencils, one wire to each graphite. 
3. Place the other two exposed graphites of the pencils into the water.
4. Add salt to the water and make your observations to the bubbles being 
formed on the graphite. You're making hydrogen and oxygen gas!

Explanation:

By placing the two graphite ends of the pencils into the water you are placing electrodes into the water because they are connected to the positive 
and negative terminals of a battery. The electrical current generated from the battery (electricity) is passed through the water from one electrode to another. In this process water (H2O of course) separates into hydrogen and oxygen. This is a process known as electrolysis.