Sea ice
Although it's not smooth like a mirror, sea ice is very shiny. Most of the sunlight that hits it bounces off and shines back into space. By not absorbing a lot of sunlight, the poles stay cold. Having cold regions near the top and bottom of the earth is important for global weather, because it affects the way cold and warm air moves around. When cold air moves down from the Arctic, it can cause very serious storms. When warm air moves up into the Arctic, it can cause the sea ice to melt and break up.