field. Blocks fall down to the playing field, and must be removed before they reach the top. If they reach the top and overflow, the game is over. In some variations, such as Bubble Bang, circles or balls are used instead of blocks—which alters the gameplay, as the balls form different shapes than square blocks. In three-dimensional variants, the playing field is a cube (containing smaller cubes) instead of a rectangle, and the player has the ability to rotate the cube. "Cubes" for iPhone OS uses this approach. Some versions allow the player to rotate the playing field 90 degrees clockwise or counter-clockwise, which causes one of two things to happen: The left and right sides become the bottom and the top, and the blocks fall to the new bottom. The orientation switches between portrait and landscape. NeoSameGame for iPhone OS uses this approach. The blocks fall to the left or right side, but the player must rotate the field back to portrait orientation (which is fixed). Bubblets Tilt for iPhone OS uses this approach. In some variations, blocks can be removed when connected to blocks of the same color diagonally, not just horizontally and vertically. Some versions introduce new types of blocks. The different types of blocks interact in various ways with the play field; for example, one type might remove all the blocks in a row. An example of this is the "Revenge mode" in PocketPop Revenge (PocketFun) for iPhone OS. Rules variations The game ends when the playing field is cleared, or if the remaining blocks cannot be removed. At the end of play, the player receives a score. When the playing field is cleared, instead of ending the game, a new level appears—usually harder, with more block types or lower time limits, or both. The condition for winning may vary between levels. Instead of clearing the whole level, for example, a certain score or a certain number of removed blocks must be reached. When the needed score is reached, in most versions the player is allowed to clear the rest of the level. If the player cannot reach the needed score—or if the timer runs out—the game ends, and the player receives a final score. In an "endless" variant, the game starts with an empty field. The blocks or balls start falling down; but if they reach the top, new blocks stop falling, so they do