For Zynga, which also owns CityVille, Zynga Poker, Mafia Wars and Words With Friends, virtual tractor sales are big business.

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These ranged from jumping and flying games to a yoga simulator and Star Wars shooter, in which players control a virtual light sabre and use hand gestures to control action on the screen.

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Zynga makes most of its revenue selling virtual items for its games such as tractors for Farmville or weapons in Mafia Wars.

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And from the initial success of Street Wars, it expects a larger share of its revenue in the future to come from sales of virtual goods in its games.

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