In the "Mario Kart" series, players compete in go-kart races, controlling one of a selection of characters from the "Mario" franchise. Up to eight participants can compete in each race, with the exception of "Mario Kart Wii", which holds twelve. One of the features of the series is the use of various power-up items obtained by driving into item boxes laid out on the course. These power-ups include mushrooms to give players a speed boost, Koopa Shells to be thrown at opponents, and banana peels that can be laid on the track as hazards. The type of weapon received from an item box is random and often influenced by the player's position in the race. For example, players lagging far behind may receive more powerful items while the leader will only receive small defensive items. This gameplay mechanic–called "rubber banding"– allows other players or computers a realistic chance to catch up to the leading player no matter how far ahead they are. As the series has progresses, each new title has introduced new elements in order to keep the gameplay fresh. Changes typically include new items, new stages, and new methods of obtaining speed boosts, but most games have also featured a new play mechanic as well. Changes new titles have brought to the series include: "Mario Kart 64" introducing 4-player racing and bringing the series into the third dimension, "'s" introducing a partner system and the Double Dash!!, "Mario Kart Wii's" use of motorbikes and twelve participants per race, and the newest entry into the franchise, "Mario Kart 7", featuring hang gliders, submarines and a new first-person perspective as well as customizing karts.