Complex sentence: For example, on the Kelvin temperature scale, zero is the coldest possible temperature (negative temperatures are defined but negative temperature systems are not actually colder), whereas on the Celsius scale, zero is arbitrarily defined to be at the freezing point of water.
Split into simpler sentences: For example, for an absolute temperature (as measured in Kelvin) zero is the lowest possible value (negative temperatures are defined but negative temperature systems are not actually colder). This is in contract to for example temperatures on the Celsius scale, where zero is arbitrarily defined to be at the freezing point of water.