Sand

Sand is a block naturally found on coasts and desert biomes, generally in four-block-deep layers supported by stone and sandstone. Sand has a very low blast resistance.

Construction
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.

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 six blocks to which gravity applies (the others being gravel, TNT (turns into primed TNT), anvil, Red Sand and the Dragon Egg). 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, rails, or Redstone), it will drop and turn into a sand resource block. Before the 1.4.3 pre-release, whenever falling sand fell into lava, it would catch fire (since it's an entity) but still landed and piled up like normal if the falling distance is short. When the sand became a block again, the fire went out; otherwise, the falling sand block burned up/disappeared while falling. 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). Falling sand can be obtained using the "/summon" command. It's Network ID is "21", and it's Savegame ID is "FallingSand".

Red Sand
Red Sand is a variation of sand that can be found in mesas. Red sand acts similar to sand, the only differences being color, and that red sand cannot be crafted into sandstone.

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.
 * If sand falls onto tall grass, rail, powered rail, flower pot, detector rail, redstone torch, torch, trapdoor, signs, 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.
 * 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.
 * Even though 4 sand blocks can be crafted into 1 sandstone, 1 sandstone cannot be crafted into 4 sand blocks.
 * Falling sand entities can be caught inside cobwebs. When falling, the block will be slowed down. If there are more blocks falling into the cobweb than there is room beneath the cobweb for the sand to settle, the sand caught in the cobweb will instead drop as items. This can be used to simulate quicksand.
 * Sand will fall through torches without breaking if there is air below the torch.
 * The block ID of falling sand can be changed so that when falling it will look like that block and will change into that block when it hits the ground
 * Sand and gravel take about 0.45 seconds to fall one meter.
 * In Pocket Edition, if sand falls onto a torch, it will not drop itself as an item, but simply disappear as soon as it comes into contact with the torch. (This was tested in 0.7.6 Alpha)