Gravity

Gravity is an In-Game mechanic obeyed by entities. It makes entities fall if there is no block below them until they hit a block or they fall into the void. All mobs obey gravity, but only 2 blocks (Sand, Gravel) can obey gravity as of Update 0.10.0. '

TRIVIA

 * Note that thown items do obey gravity.
 * Liquids also obey gravity (except milk)