A vivarium is usually made from clear container (often plastic or glass). Unless it is an aquarium, it does not need to withstand the pressure of water, so it can also be made out of wood or metal, with at least one transparent side. Modern vivariums can be as simple as a bottle (see Bottle garden) and are sometimes constructed from epoxy-coated plywood and fitted with sliding glass doors. Coating the inside of a plywood vivarium helps to retain the natural effect of the environment. Epoxy-coated plywood vivariums retain heat better than glass or plastic enclosures and are able to withstand high degrees of humidity. They may be cubical, spherical, cuboidal, or other shapes. The choice of materials depends on the desired size and weight of the entire ensemble, resistance to high humidity, the cost and the desired quality.