Stomata, or the plural stoma, are openings in the underside of a leaf on a plant that facilitate the exchange of gases into the atmosphere. These gases include carbon dioxide, oxygen, and water vapor. The stomata are placed between two guard cells which swell, causing the stomata to close, usually in times when the plant is trying to retain water. Carbon dioxide from the atmosphere enters the stomata and is used by the plant's cells in the process known as photosynthesis, the process which takes carbon dioxide and combines it with water and energy from the sun to manufacture the plant cell's food, glucose. Oxygen is produced as a waste product from the photosynthesis process, and is exited through the stomata as well.