When any object, such as a glass of water, a greenhouse or the Earth's atmosphere, stays at a steady temperature, the amount of energy entering the object is equal to the amount of energy leaving it. This is considered in equilibrium. This equilibrium changes when the amount of energy entering does not equal the amount of energy leaving. Increasing the concentration of carbon dioxide in the atmosphere means the energy leaves at a lower rate and the temperature rises -- until a new equilibrium is reached. The term 'greenhouse effect' was first used in 1901 by Swedish meteorologist Nils Gustaf Ekholm. A greenhouse is a good model for understanding the temperature of the Earth's atmosphere. In the model, the glass represents the carbon dioxide in the Earth's atmosphere. In this article, Ben Rogers summarises the history and science behind the greenhouse metaphor and gives some ideas for teaching it.