The tomatillo (Physalis philadelphica) is a plant of the Solanaceae (nightshade) family, related to tomatoes, bearing small, spherical and green or green-purple fruit of the same name. Tomatillos, referred to as green tomato (Spanish: tomate verde) in Mexico, are a staple in Mexican cuisine. Tomatillos are grown throughout the Western Hemisphere The tomatillo fruit is surrounded by a paper-like husk formed from the calyx. As the fruit matures, it fills the husk and can split it open by harvest. The husk turns brown, and the fruit can be any of a number of colors when ripe, including yellow, red, green, or even purple. Tomatillos are the key ingredient in fresh and cooked Latin American green sauces. The freshness and greenness of the husk are quality criteria. Fruit should be firm and bright green, as the green colour and tart flavour are the main culinary contributions of the fruit. Other parts of the tomatillo plant contain toxins, and should not be eaten. Tomatillo plants are highly self-incompatible (two or more plants are needed for proper pollination; thus isolated tomatillo plants rarely set fruit). Fresh ripe tomatillos will keep in the refrigerator for about two weeks. They will keep even longer if the husks are removed and the fruits are placed in sealed plastic bags stored in the refrigerator. They may also be frozen whole or sliced.