Minecraft wiki entry for Sand, associated text: If falling sand lands and covers the head of a mob or the player, the mob or player buried in it continuously receives suffocation damage. If falling sand lands in the space occupied by a non-solid block (such as torch, slab, rail, or redstone dust), the sand drops and turns into an item. If it falls onto a cobweb, it falls slowly until it has gone through completely; if it touches the ground while still inside the cobweb, the sand becomes an item. Sand that falls onto a lifting bubble column floats on top of the water until the bubble column is blocked or removed.