Causes
1. Heat
2. Pressure
Heat in Physics is an energy that is in a process of transfer between a system and its surroundings other than the transfer of matter. In Thermodynamics, a science that deals with the study of heat and temperature and their relationship to energy and work, finer details of the process is unpredictable. Heat flows from a hotter to a colder body when there is a physical pathway. This pathway is suitable and can be direct such as radiation and conduction. It can also be indirect as in convective circulation.