Pokémon are creatures found in the world of Pokémon that are typically raised as pets or use as a fighting tool. With any living thing, the more you train the bigger and stronger you become. Pokemon who frequently train with their Trainers or gain experience in battles can evolve and transform into a more powerful version. Gaining more strength and learning new skills to aid them in battle.
Many Pokemon grow and evolve naturally, but some require specialised items to help them transform and evolve. With the introduction of Gen 4 Pokémon in Pokémon Go, to evolve certain Pokemon you need a Sinnoh Stone.