Different Temperature Scales
The temperature scales have different starting points and are named after their inventors.
Celsius and Fahrenheit
The scales have a zero point and are graded in both plus and minus degrees.
The thermometer reading is a measurement of the kinetic energy of the molecules (= the heat).
Because there is a thermometer reading also at minus grades, the molecules cannot have already stopped moving (heat be zero) at 0°
In terms of pure physics, it is also incorrect to talk about warm and cold grades. Put simply, different temperatures correspond to different levels of heat.
Celsius took water as a starting point when he made his scale.
The boiling point of water was set at 100° and the freezing point at 0° on his scale.
Fahrenheit cut his finger and used both his blood temperature and a refrigerated mixture of ice and salt as his starting points.
The temperature of his blood was set at 100° and the frozen mixture's temperature was set at 0° on his scale.
Conversion between Celsius and Fahrenheit:
C = ((F-32) x 5) / 9
F = (C x 9) / 5 + 32
Kelvin
This scale is based on the kinetic energy of the molecules.
0° K = absolute zero = the molecules have stopped moving completely.