The player begins with a certain number of lives that represent the number of times the player is allowed to lose all of his health (and then re-spawn immediately from the player's last saved checkpoint). More complicated gameplay mechanics that are often found in other games (such as jumping puzzles) are rare, and when encountered they are fairly simple, usually requiring the player to locate keys/objects in order to unlock doors or advance to the next level. Player-controlled vehicles (such as hover bikes and saucers) were introduced to the series in "Serious Sam 2". Vehicles feature turrets such as rocket launchers, machine guns, and laser turrets.