A domino is a small rectangular piece of wood or plastic marked with dots that resemble those on dice. A set of these pieces, usually 28 in number, is used for playing a variety of games. The game involves laying tiles down in a line and then “building” upon them with the aim of making long chains. A player cannot play a tile that has another tile’s number showing on both ends; this is called “stitching up the ends.” Traditionally, domino sets are made from materials such as bone, silver lip ocean pearl oyster shell (mother of pearl), ivory or dark hardwoods such as ebony, with black or white pips inlaid or painted. More recently, sets have been produced from other natural substances such as marble, granite and soapstone; metals such as brass and pewter; ceramic clay; or glass and crystal. The most common type of domino game is played with a double six or double nine set, although larger sets are available for players who wish to play longer domino games. The most popular types of domino games are blocking and scoring. Blocking games require the players to make a series of moves that prevent the opponent from playing any more tiles; scoring games involve accumulating points as one player plays all the tiles in his hand. Some games are hybrids and combine elements of both blocking and scoring. While domino is typically played in a group, it can also be enjoyed alone. Some games are designed to be played in pairs, with one player taking turns at a time. Other games are played only between a single player and the computer. In addition to the skill of the players, a great deal of luck is required. A good strategy is important, but so are quick reactions. Players who are slow to respond or who fail to take advantage of opportunities may find themselves behind the rest of the field. A good strategy is to draw a large number of tiles for your hand and then carefully choose the ones to play, as explained in the rules for each particular game. It is important not to “overdraw,” that is, draw more than the number allowed for your hand. A player who draws more than he is entitled to must return the extra tiles to the stock and then reshuffle before anyone else begins drawing for their hands. When a player is unable to play any of the dominoes in his hand, he must announce it to the other players and then knock, or rap the table, with the dominant domino. A successful rapping can cause the other players to chip out, thereby ending the game. Physicist Stephen Morris explains how this occurs: When a domino is standing upright, it has potential energy, which is the energy stored in its position. But when the domino falls, much of this potential energy is converted into kinetic energy, or the energy of motion. This is what causes one domino to topple after the other.