MOCathalon is an annual LEGO building contest with a series of categories. One of this year's categories is 'Just teach it!', and Finn Tegotash built a chemistry class.
Okay, class, we're going to continue with our examination of the periodic table.
Hydrogen is the simplest element, with only one proton and one electron. In molecular form it exists as H2.
Since it is the lightest element, and since gases take up the same volume of space regardless of their mass, the same volume of hydrogen is much lighter than the same volume of air (a mix of nitrogen, oxygen, carbon dioxide, water vapor, and some other things). Therefore, a balloon filled with hydrogen floats.
Hydrogen reacts with oxygen to make water. This reaction gives off a lot of energy. It is an exothermic process. Boom!