Terrariums are usually sealable glass containers that can be opened for maintenance and to access the plants inside. However, this is not essential; terrariums can also be open to the atmosphere rather than being sealed. Terrariums are often kept as decorative or ornamental items. Closed terrariums create a unique environment for plant growth, as the transparent walls allow for both heat and light to enter the terrarium. The sealed container combined with the heat entering the terrarium allows for the creation of a small scale water cycle. This happens because moisture from both the soil and plants evaporates in the elevated temperatures inside the terrarium. This water vapour then condenses on the walls of the container, and eventually falls back to the plants and soil below. This contributes to creating an ideal environment for growing plants due to the constant supply of water, thereby preventing the plants from becoming over dry. In addition to this, the light that passes through the transparent material of the terrarium allows for the plants within to photosynthesize, an important aspect of plant growth.