If you ask a student to name the largest desert in the world, they will almost certainly say the Sahara. However, in geographical terms, a desert is defined not by heat or sand, but by precipitation (or lack thereof). A desert is any region that receives less than 10 inches (250 mm) of rain per year. Under this scientific definition, the largest desert in the world is the Antarctic Desert, which covers 5.5 million square miles. The Sahara, while being the largest *hot* desert, is only the third-largest desert overall, following Antarctica and the Arctic Desert. This fact is a classic trick used to test whether someone understands the meteorological definition of a desert biome.