Everyone knows sand. Maybe you played with it as a kid, maybe you built castles on the beach, maybe you use it to make your walk less slippery. It is everywhere. There are seas of sand on our planet. It can be tan, white, green, black and more. Sand is economically valuable. To an Earth scientist, sand is also a vault, containing the history of long destroyed mountains, a changing climate and vast tectonic collisions.
Sand is a surprising specific thing, at least in the Earth sciences. For something to be sand, it has to be between 0.063 and 2 millimetres in diameter. Anything smaller is mud, anything larger is gravel, cobbles or boulders. Sometimes sand is all you will find, like in a desert. Sometimes that sand is mixed with pieces of varying sizes, like on a bar along a river.
