0 degrees centigrade is the changing point when ice becomes water and water becomes ice. They are the SAME thing.

The problem is that you think that temperature is the unit of measurement of how much heat is present in something; it is not. Temperature is the result of adding or subtracting heat, measured in Calories, to or from anything. (Side Note: Please do not confuse the Scientific Calories of which I speak with the Dietetic Calories that is the measure of energy that food gives a human when digested. 1000 Scientific Calories = 1 Dietetic Calorie.) Please let me try to explain what happens when you add heat to a substance (this includes ice-water-steam ): If you start by adding calories to a solid substance (for example: ice) the temperature increases in a straight line relationship to the amount of calories. As the temperature of the substance reaches the melting point, the temperature stops increasing, until all of the substance has been melted. The amount of calories that you must add to melt all of the substance is called "The Enthalpy of Fusion" http://en.wikipedia.org/wiki/Enthalpy_of_fusion. After you have added the amount of calories of fusion, all of the substance is a liquid (for example: water) and the temperture resumes increasing in a straight line relationship with respect to the amount of calories added. As the temperature of the substance reaches the boiling point, the temperature stops increasing, until all of the substance has been turned into a vapor (for example: steam). The amount of calories that you must add to vaporize all of the substance is called "The Enthalpy of Vaporization" http://en.wikipedia.org/wiki/Enthalpy_of_vaporization. After all of the liquid has been turned into a vapor, the temperature resumes increasing in a straight line relationship to the amount of calories added. When any substance cools, the reverse of the above process happens. The Vapor cools, until the boiling point, then the temperature does not change until all of the substance becomes liquid and so on. I hope that this helps your understanding of heat versus temperature.