The angle of repose or critical angle of repose is how steep various types of materials can be piled without slumping. The steepness of the side of a pile is determined by the amount of friction between the particles of matter. This can vary according to size and density of the material, particles size, smoothness of the particles, whether it is wet or dry, and any cohesive properties of the matter. Materials such as sand piles, dirt, sugar, flour, stones and every other type of loose matter will all exhibit this property. Here are a couple of devices that were made for demonstrating this process using sand without it making a mess.