Jacks are playing pieces used in the children's game of the same name. The game of jacks has been played for thousands of years in cultures all over the world, and children today continue to play it, evolving their own complex rules for the game. Most game stores carry sets of jacks along with the ball needed for play, although it is also possible to improvise the game pieces used in jacks with stones, sticks, bones, and other objects which can be found lying around the house of garden.

The game of jacks gets extremely complex, but the basic object of the game is to pick all of the jacks up off the ground while the ball is tossed in the air, and people play with anywhere between five and 15 jacks. When people play jacks, they start with “onesies,” in which one jack at a time is picked up with each toss. If someone completes onesies without a mistake, he or she moves on to twosies, in which two jacks at a time are picked up while the ball is in the air, and then on to threesies, and so forth.

Play turns to the next player once someone makes a mistake such as dropping jacks, failing to pick up the required number of jacks before the ball hits the ground, or violating a rule of the game as determined by the players. The game continues in a circular manner until someone has picked up all of the jacks at once in one toss, although this can be extremely difficult with large numbers of jacks.

As anyone familiar with division may have noted, the numbers of jacks to be picked up in each round do not always divide evenly into the total number of jacks. This means that the player has leftover jacks which must be picked up with the last toss of the ball. A player can also opt to pick up the “remainder” midway through the game, although he or she must declare the intent to do so.

Advanced jacks players may challenge each other to various complex physical feats with the jacks or the ball, making the game more difficult. Many people are familiar with their own arcane jacks rules, many of which have colorful slang terms attached, which can make playing jacks with a new group frustrating.

This game is also known by a wide variety of other names, including pigs in the pen, five stones, five finger, knucklebones, eggs in the basket, cincos marias, abhadho, and so forth. The appeal of jacks in the eyes of many people is that it requires no special equipment, and children can play it for hours. Games of jacks are an especially common sight in the developing world, where children tend to lack access to toys and fancier children's games.