Sand

Sand is a block naturally found on coasts and deserts. Sand has a very low blast resistance.

Red Sand is a variation of sand that can be found in mesas. Red sand acts similar to sand, the only differences being color.

Obtaining
Sand generates naturally in many biomes of the Overworld, notably deserts, beaches, and rivers, generally in four-block-deep layers supported by stone and sandstone. Red sand generates naturally in Mesa biomes and variants.

Sand can be mined instantly with an Efficiency III or better diamond shovel.

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.

Farms
Sand and red sand can be used for farming cacti and sugar canes.

Behavior
Sand, if there is no block below it, will 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 the primed TNT entity), 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 item. 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. Additionally, if sand is placed anywhere straight up from a painting entity or an item frame entity, it will pass right through the painting or item frame like any other entity (the same happens to gravel).

Minecraft is capable of making any kind of a block become a falling block. Unlike usually thought, there is only one FallingSand entity. It is able to take any texture of any block,even non-solid blocks, such as a torch, but doing that will usually mess up the textures. FallingSand has two important NBT Tag fields that change the appearance, movement, and lifetime of the entity. Most notable of the is BlockID. Using a BlockID for FallingSand in /summon grants the player the ability to, for example, create a falling bedrock block. ('/summon FallingSand ~ ~ ~ {BlockID:7}'). Note that even though as of 1.8 Minecraft has moved to item ID's of 'minecraft:stone' etc, the BlockID still uses the number format. The other important tag is Time. Time stands for the time (in ticks) for how long the FallingSand has existed. If 0, the entity despawns, if 1 or greater, will continue ticking up, until it reaches a maximum lifetime. This means if the player wants to have a falling bedrock block that does not despawn immediately, then they should put that in the NBT tags ('/summon FallingSand ~ ~ ~ {Block:"minecraft:bedrock",Time:1}'). When the maximum lifetime is achieved, the FallingSand will despawn and drop a resource Block respective to the Block. In normal conditions the maximum lifetime will not be reached, but using a command that would make a FallingSand ride another entity, would stop it from falling, and therefore from becoming a normal block. If this is done, the Time will be ticking up until the maximum lifetime is reached.

Data values
Sand has the ID name  and is further defined by its block data. Sand also has a block state which is expected to replace the functionality of block data in a future version.

Trivia

 * If a block of sand has snow on it and is made to fall, the snow is destroyed and will not drop snowballs.
 * Even sand is affected by cobwebs. If it falls on one, it will slowly fall and eventually 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.
 * 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.
 * Sand and gravel take about 0.45 seconds to fall one meter.