Hydrogen Gas
Hydrogen gas is the lightest gas in the world, and you can make it by running electricity through water.
Think of it this way
Put two pencils in salt water and connect them to a battery. Tiny bubbles form on each pencil tip. The bubbles on one pencil are hydrogen gas, the lightest gas that exists. It floats up through the water and vanishes into the air. The electricity split the water apart, and the hydrogen escaped as those tiny rising bubbles.
Explaining hydrogen gas by grade level
When you put two pencils in salt water and connect them to a battery, tiny bubbles appear. Those bubbles are hydrogen gas floating up and away. Hydrogen is so light it rises faster than any other gas. It has no color and no smell, so you can only see it as bubbles in the water.