Just fill a glass bottle and find total volume. Divide by 3, and fill with that much water. Mark that spot as it will be 1/3, This will be the for the oxygen since it combines in a 1:2 ratio with hydrogen. The other 2/3 are of course for the hydrogen. Simply fill the whole container with water and invert in water. Let pure oxygen bubbles displace the water as they rise into the container until the gas fills up to the 1/3 line. Then switch to hydrogen gas to fill the remaining space. Plug the hole before removing, then light the gas with a flame source. Bang. This makes water and shows the meaning of the balanced equation of making water and the chemical formula of water.