Sand

Sand is a block naturally found on coasts and Desert biomes, generally in four-block-deep layers supported by Stone and Sandstone. Sand blocks can be mined easily by hand or with shovels and drop as resources regardless of the tool used. Sand has a very low blast resistance.

Behavior
Sand, if there is no block below it, will turn into Falling Sand and fall until it lands on the next available block, thus making it one of four blocks to which gravity applies (the others being gravel, Dragon Egg and Activated TNT). This will happen even if the block of sand is attached to a Sticky Piston, as the piston will only stick to a block when retracting. When Sand is being affected by gravity and falling, it exhibits a smooth falling animation. If falling sand lands on a mob or the player and covers their head, it will suffocate them until they successfully destroy the block, move out of the block or die. If Falling Sand lands in the space occupied by a non-solid block, (such as Torches, Slabs, Minecart tracks, or Redstone), it will drop and turn into a sand resource block. If Falling Sand falls into Lava, it will catch fire but still land and pile up like normal. When the sand becomes a block again, the fire goes out. Also, if it falls onto a cobweb it will slowly fall until it has gone through it completely, or until it touches the ground, at which point it will turn into a resource block. Placing a block of sand into the space directly above a non-solid block will not cause it to turn into Falling Sand. Sand blocks can be suspended in indefinite defiance of gravity in this way, most famously using torches. Additionally, if sand is placed anywhere straight up from a painting, it will pass right through the painting as if it wasn't there (the same happens to gravel).

Current Uses
Sand can be used in the construction of Airlocks and the creation of mob suffocation traps. It is also useful as a scaffolding material in some situations. Being affected by gravity, it can be quickly and easily removed from ground level and can be used to construct a safe path downwards from a higher position (such as a hole directly down through the roof of a large cavern) even if the ground itself cannot be reached by hand.

In addition, sand is used in the production of glass via smelting and TNT via crafting with gunpowder.

Torch Mining
When mining a large column/pillar of sand, a good method is to dig under the stone or dirt that it is resting on. Under that dirt or stone, place any partial block (such as a torch, slab or cobweb). Mine the dirt or stone and the column falls into the placed object, quickly destroying the sand. This can also be used to mine Gravel. Leaves can break sand/gravel.

Cofferdam mining
If large quantities of sand and sandstone are needed, the ocean floor is a great source of both of these. To create a cofferdam, find a place in a large lake or ocean that is only one or two blocks deep, and fill in that spot to be just above water level. Then dig a pit inside so that the water can't get back in. With a large enough pit, a large chest can be filled with sand and sandstone fairly easily.

This method can also be used in Desert biomes without the need to drain away water.

History
Sand has been available for use since Classic Mode. During that time, Sand blocks were rare and only appeared naturally in one block-thick beaches (usually by water or in the middle of a landscape). These beaches were always at and below Ocean level. Sand instantly appeared at the lowest point it could go when placed above an air block without any sort of falling animation. Destroying a pillar made from sand from the bottom would cause the sand to disappear from the top first. Gravel and sand "fell" when placed in mid-air by moving directly on top of the nearest block directly below them, instead of turning into falling block entities and falling. These attributes were present until the release of Infdev. Code for this still exists within the source of Minecraft, in the form of a boolean dictating whether it does this or exhibits its new behavior.

Map editors could be used to create floating Sand, although the server would crash if the Sand was affected from its state.

Before Beta 1.3, generated sand did not have Sandstone under it, and if a cave spawned below sand, then floating sand blocks would persist until one of the blocks was updated.

An old glitch in Classic mode allowed players to raise the height of a fluid block by placing Sand (or Gravel) over it. The Sand block would stay suspended in mid-air until it was broken. When broken, a fluid block corresponding to the type below the Gravel would appear where the block was. The suspended fluid block would remain immobile until a block was placed next to it, causing a flood. This bug has since been fixed.

Originally, sand would not turn into an item object when falling onto a non-solid object, thus destroying itself.

Trivia

 * In classic, if the player is next to a set of blocks and places 2 sand blocks above them, using the side of those blocks, they will end up inside of those blocks, with an internal texture identical, but darker than regular sand. Like most blocks, they are also hollow. The same bug happens with gravel.
 * If a block of sand has snow on it and is made to fall, the snow is destroyed and will not drop a snowball.
 * Sand does not fall in Minecraft: Pocket Edition.
 * If sand falls onto tall grass, rail, powered rail, detector rail, redstone torch, torch, trapdoor, flowers, redstone repeaters, or cobwebs it will break.
 * Because falling sand is considered an entity it can be launched in a TNT cannon, similar to an ignited piece of TNT. The only significant building difference between a normal TNT cannon and a sand cannon is that a piston is needed at the end of a sand cannon. A correctly-timed piston will allow a sand block to fall just before the TNT charge detonates, launching the falling sand entity a considerable distance.
 * There is a bug with this which will duplicate the sand. When using 2 TNT's timed almost after each other and placing the sand in air just before the TNT explodes, the sand will duplicate into 2 sand blocks on impact.
 * Sand will fall at the same rate when submerged in water as it will when in air. This is also true for lava.
 * Starting in 1.8(Adventure Update) with the new seed terrain coding, floating sand is more common and if you break or place a block close to it it will then begin to fall.
 * If a player is standing on a stack of sand or gravel, and the stack falls on a non-solid block, the player will fall fast enough to take damage or even die.
 * As of 1.8, sand generates anywhere that water does in large, circular patterns, noticeably larger than the similar patterns of clay. These can occur anywhere water does, including in NPC Village farms. Before beaches were re-added in 1.1, this would create what looked like a small beach along shorelines.
 * When sand falls 117 blocks it will turn itself into an item, even if it would land on a solid block.
 * Even though 4 sand blocks can be crafted into 1 sandstone, 1 sandstone cannot be crafted into 4 sand blocks.