Simplify the following text for a Grade 8 native English-speaking person.
Original: When reference is made to the unit kelvin ( either a specific temperature or a temperature interval ) , kelvin is always spelled with a lowercase k unless it is the first word in a sentence .
Simplified: The Kelvin scale is defined by a specific relationship between the pressure of a gas and the temperature . This says that `` the pressure of the gas is directly proportional to the temperature in Kelvin '' .