with a Wok in China". Credit: The Pocket . Linked source: Wikipedia image http://en.wikipedia.org/wiki/Image:Wok_cooking_and_the_heat_source_by_The_Pocket_in_Nanjing.jpg . Licensed under the Creative Commons Attribution 2.0 . Also a system 's size changes thermal energy is added or subtracted: usually it increases when thermal energy is added and contracts when it is removed---but NOT always.