Most people assume that a desert is defined by heat, but climatologists classify deserts strictly by scarcity of precipitation, not by temperature. By this measure, the largest desert on Earth is not the Sahara but Antarctica, whose interior receives less moisture in a year than many places in the Arabian Peninsula. The confusion arises because our everyday image of a desert—dunes shimmering under a brutal sun—is drawn from the hot deserts of popular imagination. Yet aridity, not warmth, is the trait that unites a frozen polar plateau with a baking sand sea. Once the definition is made precise, familiar intuitions about what a desert must look like quietly collapse.