Simplify the following text for a Grade 6 native English-speaking person.
Original: In the study of heat , both in engineering and chemistry , enthalpy ( written as H ) is a useful measure that scientists have developed to find out how much work has been done , or heat absorbed , when a gas changes its size at a constant pressure .
Simplified: Heat is not the only way to change enthalpy . Enthalpy also changes when the pressure of the environment is altered , even if no energy is exchanged as heat .