A joule is a unit of measurement for energy, named after James Prescott Joule, a 19th-century English scientist who studied heat and energy. One joule is roughly the amount of energy it takes to lift a small apple one meter off the ground, or the energy released as heat when a one-watt lightbulb runs for one second.

Scientists use joules to measure all kinds of energy: the energy in your moving bicycle, the energy stored in a battery, the heat energy that warms your home, or the electrical energy flowing through your phone charger. A nutritional Calorie (the kind on food labels) equals about 4,184 joules of energy, which your body converts into movement, heat, and everything else that keeps you alive.

The joule helps scientists compare different types of energy using the same standard. Whether measuring the kinetic energy of a soccer ball flying through the air or the potential energy stored in a stretched rubber band, joules give us a common language. When you see megajoules (millions of joules) or kilojoules (thousands of joules), scientists are measuring much larger amounts of energy, like the energy in a lightning bolt or the fuel in a car's gas tank.