Air, though technically not a block, is usually treated like such. Air is literally the absence of a block; It represents unoccupied spaces. Air can only be destroyed or created by the placement or removal of blocks, respectively. Air and non-solid blocks are the only spaces the player and other mobs are allowed to move within without mods or glitches. The sole exception is the Ender Dragon who can move within Obsidian, Bedrock, and End Stone.