Items in free games might be purely aesthetic, increase the player's strength, quicken the rate of progression, and much more. The items the player buys in these games often have a limited lifespan, and once that time is up, the player must buy the item again in order to proceed. Another common feature is the presence of two different currencies: one obtained naturally via play and one bought with real money. Non-paying players can earn a tiny amount of the second currency (called "premium" currency) in a variety of ways, including starting a new game, completing a quest, and referring a friend. There is typically some kind of "energy bar" in web games that is depleted when the user does things. Then, to keep the bar stocked, these games sell consumables like coffee and snacks.