Although the game is free to play, it supports in-app purchases, where players can purchase additional Poké Balls and other in-game items. These items include Incense (which attract Pokémon to you as you move for thirty minutes), Lure Modules, which you have to use at PokéStops to attract Pokémon to your current location near the PokeStop, and Lucky Eggs, which double experience points gained for a thirty-minute period from use. All Pokémon are displayed with a Combat Power, which is a rough measure of how powerful that Pokémon is in battle. Generally, as players level up, they catch Pokémon with higher CP and Pokémon are somewhat harder to catch. The player can check how strong their Pokémon are by the “Appraisal” system.