The Antarctica is actually a desert. It is the only continent on the earth without a river or a lake. The Antarctica is all ice all year round. The warmest temperature ever recorded there is zero, at the South Pole. Explorers used to think that a place so cold would have a heavy snowfall. But less than ten inches of snow falls each year. That is less than half an inch of water. Ten times that much moisture   falls in parts of the Sahara. The little snow that falls in Antarctica never melts. It continues to pile up deeper and deeper year after year and century after century. When the snow gets to be about eighty feet deep it is turned to ice by the weight of snow above it . The snow turns to ice when  _  .
it gets wet. the next snowfall comes. the temperature gets colder. the snow above it is heavy enough.