A piston is displayed in a cylinder. The piston is near the bottom of the cylinder with a small cushion of air between the piston and the bottom of the cylinder. Flames heat the underside of the cylinder. The air between the piston and the bottom of the cylinder begins to turn slightly orange to illustrate that the air is becoming warmer. As the air warms, it expands, pushing the piston upward. An arrow pointing to the warming air is labeled, “Heat (flames) causes the pressure inside to increase, pushing up the piston.”