Heat causes ice to melt. Ice returns to the liquid state when the temperature rises above the freezing point of water, which is 32 degrees Fahrenheit or 0 degrees Celsius. Additionally, because of the specific and unusual characteristics of water, increased pressure causes it to melt as well.
As with most substances, water can exist in at least three states of mater. If water is warmer than 212 degrees Fahrenheit, it takes the form of a gas, and is called water vapor. If it cools to below this temperature, it condenses into the liquid state. If the water continues to cool, it eventually turns into a solid called ice.
Some substances can lower the freezing temperature of water, which will make ice revert to its liquid form. Salt is one such chemical, which is why municipalities cover frozen roads with it. As long as the temperatures are above 15 degrees Fahrenheit, the salt melts most of the snow or ice.
Unlike most substances that take up less space when they freeze, ice takes up more space than an equal amount of water does. When pressure is applied to ice, it resists collapsing and melts instead. In some very specific conditions, water can be cooled below the freezing point.