The standard unit for measuring energy, work, and heat in the metric system used by science everywhere — its symbol is a capital J. One joule is a small amount: roughly the energy it takes to lift a small apple one metre, or to run a one-watt light for one second. Because it is so small, real life uses bigger versions — kilojoules (kJ, a thousand) and megajoules (MJ, a million). Students meet it constantly in physics, since work, heat and every kind of energy are all measured in the same unit. You also meet it on food packets: outside the United States, nutrition labels give energy in kilojoules, sometimes alongside calories. One food calorie is about 4.2 kilojoules, which is why the kilojoule numbers on a packet look so alarmingly large. The unit is named after James Prescott Joule, the English scientist who showed that heat and mechanical work are two forms of the same thing.