* Fahrenheit (°F) and Celsius (°C) are both interval scales. This means:

* They have arbitrary zero points. For example, 0°C doesn't represent the absence of temperature, it's just a defined point on the scale.

* The size of a degree is defined, but the actual amount of heat it represents is not absolute. A 1°C change doesn't represent the same amount of heat as a 1°F change.

* Because of this, you can add and subtract temperatures on these scales, but you can't multiply or divide them.

* Kelvin (K) is an absolute scale. This means:

* It has a true zero point. 0K represents the absence of all thermal energy (absolute zero).

* The size of a Kelvin is the same as a Celsius degree, but it's based on a physically meaningful zero point.

* This allows you to perform any mathematical operation on Kelvin values because the values are truly proportional to the energy present.

In summary:

* Fahrenheit and Celsius are relative scales that use degrees to measure temperature differences.

* Kelvin is an absolute scale that uses a unit called the Kelvin, representing an absolute measure of thermal energy.

This is why we don't say "degrees Kelvin". It's simply Kelvin.