One distinctive game mechanic of "Sonic" games are collectible golden rings spread throughout levels, which act as a form of health. Players possessing rings can survive upon sustaining damage, but the rings are scattered and the player has a short amount of time to re-collect some of them before they disappear. Collecting 100 rings usually rewards the player an extra life. Rings have other uses in certain games, such as currency in "Sonic '06", restoring health bars in "Sonic Unleashed", or improving statistics in "Sonic Riders". Levels in "Sonic" games feature elements such as slopes, bottomless pits, and vertical loops. Springs, springboards, and dash panels are scattered throughout and catapult the player at high speeds in a particular direction. Players' progress in levels is saved through passing checkpoints. Checkpoints serve other uses in various games, such as entering bonus stages. Some settings, most notably Green Hill Zone, recur throughout the series.