A quantity of things heaped together, this defines a pile. Say money or rubble. Could be inorganic matter like maybe sand or organic material such as leaves or corpses or crullers. Like things or unlike things. For instance garbage could be all different kinds of stuff but what it has in common is it's all been discarded, and therefore it forms a pile of discarded things, all different but united under the pile of garbage banner. Not necessarily things, could be problems, i.e. you could go metaphorical with it. A pile of problems conjures a certain amount of disorder, much different than an ordered stack of problems. You'd have your head in your hands, lamenting this pile of problems in your inbox, your to-do list, and you with such an awful hangover to boot. Tequila. A stack might suggest not problems so much as challenges or tasks, you can envision handling them one by one systematically, even cheerfully, whereas a pile of problems carries a hint of the unwieldy and unmanageable. A stack of something could easily become a pile of something, like debris, almost anything can be turned into debris, such is the something something law of thermodynamics. You wouldn't have a stack of debris. BUT, think of a hurricane that just happened to leave an assortment of objects in a stack configuration as if intelligently arranged. This must have happened, it's bound to have happened, according to the something something law of probability. Then everyone in the trailer park comes to look and it becomes lore, lore of the Panhandle, undoubtedly with a gauzy religious dimension, since this is America. God spared our trailer and left this majestic calling card, a perfectly vertical stack of Toyota Corollas in ascending order of model year. Hallelujah! Come one, come all, lay hands upon the stack, let God's Corolla Stack heal your soul, or your piles.