273 + degree Celsius = Kelvin. Why 273? Temperature conversion:
$$273 + \text{degree Celsius} = \text{Kelvin}$$
Actually why is that $273$? How does one come up with this?
My teacher mentioned Gann's law (not sure if this is the one) but I couldn't find anything related to this, which law should it be?   
 A: One Celsius (or Kelvin) degree as a temperature difference was defined as 1/100 of the temperature difference between the freezing point of water and boiling point of water.
We call these points 0 °C and 100 °C, respectively. The number 100 arose because we're used to numbers that are powers of ten because we use the base-ten system. The Celsius degree is "one percent" of the temperature difference.
There also exists the minimum temperature that one may achieve. In Kelvin degrees, it's sensible to call this minimum 0 K: that's a primary condition on the "natural" temperature scales such as the Kelvin scale. If one insists that one Kelvin degree is equally large as one Celsius degree as a unit of temperature difference, it just turns out that the melting point of ice has to be 273.15 K and the boiling point of water is 373.15 K. It's because the temperature difference between the boiling and freezing points is 2.7315 times smaller than the temperature difference between the minimum allowed temperature, the absolute zero, and the freezing point of water.
This number 2.7315 can't be explained in simple words. It is a fact about water, a fact of Nature that may be derived from the laws of quantum mechanics. One has to "simulate" what the water molecules are doing all over this large interval of temperatures and this is just what comes out. We know it is so from experiments.
A: The Kelvin temperature scale is an absolute temperature scale.  That is 0 K is absolute zero.  It also has the property that temperature intervals on the Kelvin scale are the same as on the Celsius scale.  That is a decrease or increase of one Kelvin is the same as a decrease or increase of one degree Celsius.  To meet these two requirements it is necessary to choose 273.16 K as the temperature of the triple point of water.  According to Halliday and Resnick, "Fundamentals of Physics" second edition, 273.16 K was adopted as the temperature of the triple point of water at the Thirteenth General Conference of Weights and Measures in 1967 at Paris.
A: Because it is the Celsius value at which all molecular moverment stops, aka absolute zero or 0 in kelvin scale
http://www.beyondmech.com/physics-mechanis/pm-topic-5.html
