A bonus is a special feature of the particular game theme, which is activated when certain symbols appear in a winning combination. Bonuses and the number of bonus features vary depending upon the game. Some bonus rounds are a special session of free spins (the number of which is often based on the winning combination that triggers the bonus), often with a different or modified set of winning combinations as the main game and/or other multipliers or increased frequencies of symbols, or a "hold and re-spin" mechanic in which specific symbols (usually marked with values of credits or other prizes) are collected and locked in place over a finite number of spins. In other bonus rounds, the player is presented with several items on a screen from which to choose. As the player chooses items, a number of credits is revealed and awarded. Some bonuses use a mechanical device, such as a spinning wheel, that works in conjunction with the bonus to display the amount won.