Leaves come in a variety of sizes and shapes but they all have a common function: they make food using a process called photosynthesis, which is unique to green plants. When photosynthesis takes place, the plant gets water through its roots (1), which then rises to its leaves (which evergreens have all year). Here in the leaves in the presence of a green pigment called chlorophyll (2) and in the presence of energy provided by the sun (3) water is split into hydrogen and oxygen. Meanwhile, carbon dioxide enters the underside of the leaf through its pores which are called stomates (4). Next a process which we really don't understand fully takes place. Hydrogen gas is created by the splitting of water, and it is combined with the carbon dioxide that has come into the leaf through its stomates. When this process is complete, we have a sugar that contains carbon, hydrogen, and oxygen (5). The leftover oxygen is released into the air, which helps replace the oxygen in the air that we and all other animals need to breath (6).

In very cold weather, such as winter, and when the sun is not as bright and the water in the ground is frozen, not nearly as much oxygen is given off as in the summer when the sun is bright and the water in the ground is not frozen.