Deserts are characterized by little vegetation and rain, consisting of sand or rocks. They cover about one-fifth of the world's landmass and are typically found near the Tropics of Cancer and Capricorn. Oases form within deserts, providing water and allowing for plants and even settlements due to trapped water beneath the desert floor. Desert climates experience extreme heat during the day and cold at night with infrequent but heavy rainfall. Plants and animals have adapted to the harsh desert conditions, with plants developing long roots or storing water and food underground, and animals only active at dawn and dusk to avoid the heat.