"You can think of a glacier as a freely moving river of ice and snow. It forms in mountains and valleys when snow is able to accumulate over time. As more layers of snow pile up on top of another, they pack together to form ice. Slowly the ice becomes thicker and heavier until eventually the weight of so much ice and snow causes the glacier to flow downwards and move. People have seen glaciers moving in many parts of the world, even in Mexico and parts of Africa! They're also very important in studying climate change, and they're our greatest source of freshwater."