When a substance burns, it combines with oxygen and releases heat and light — but not all fuels react at the same speed. The dye mixed into candle wax might change how fast it melts. You line up five candles that differ only in color, mark a line one inch below the wick on each, and light them all at once. Timing how long each takes to burn down to that mark shows whether color alone changes the reaction speed.

Combustion needs a steady supply of oxygen to keep a substance burning. In this setup, you place a peanut inside a large metal can with air holes that acts as a chimney. Those holes let oxygen reach the peanut so it keeps burning.

Combustion releases different amounts of heat depending on the substance that burns. You burn equal-sized pieces of oak, maple, cedar, and pine one at a time under a beaker of water on a tripod stand and record the water temperature every minute for five minutes. Hardwoods like oak and maple heat the water faster than softwoods like cedar and pine.

The amount of fuel available determines how much heat and light a reaction releases. Hydrogen gas combines with oxygen when ignited, and the volume of gas trapped inside a soap bubble controls the intensity. Using a tank of hydrogen, you blow bubbles ranging from less than 1 mm to over 18 mm across and ignite each with a lighter. Bubbles smaller than 4 mm produce no visible flame at all. As size increases past that threshold the flames grow larger, and above 18 mm the combustion turns explosive.

Combustion converts solid wax into heat and light as the fuel combines with oxygen. The chemical makeup of the wax determines how fast this reaction consumes fuel. Fifteen candles of the same diameter and length from the same maker all lost about 27 millimeters in 30 minutes, whether unscented, rose-scented, or apple-scented.