Pokémon gain experience points, usually called EXP, from battles and certain other activities. When enough EXP is accumulated, the Pokémon gains a level and its stats increase according to its species, IVs, EVs, Nature, and game rules. Different species can belong to different experience-growth groups, so they may need different amounts of EXP to reach the same level. Trainer battles often provide more useful experience than random encounters, and modern games may offer candies or shared experience systems. Leveling improves general strength, but moves, typing, items, and strategy remain important against difficult opponents.