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.
Related reading: Angle of repose of various materials.Then watch more DIY physics videos: Make Lissajous patterns with DIY sand pendulums, how to collect black sand with a magnet, speed up geologic time with a DIY squeeze box, and how to make a giant fluidized bed of sand.