That is a very good question! When we talk about temperature, what we really are measuring is the energy (and speed) of the atoms/molecules within a material. The atoms move faster in gases than liquids, and faster in liquids than solids. As you cool down a solid, the atoms move slower and slower, until they reach a theoretical minimum where the atoms are not moving altogether. The atoms have ZERO kinetic energy. This temperature is called absolute zero, and is the same for all materials! The temperature is at -273 degrees Celsius or 460 degrees Fahrenheit below zero!!! This was first correctly described by Lord Kelvin, and so we call the temperature scale where absolute zero is 0 and water freezes at 273.15 degrees the Kelvin scale.

Hopefully that helps.