Simplify the following text for a Grade 5 native English-speaking person.
Original: Pikachu evolves into Raichu via the use of a Thunder Stone ; however , it is somewhat common for trainers to choose not to evolve their Pikachu .
Simplified: It can evolve ( change its type ) into Raichu , which is bigger and stronger . A pre-evolved form of Pikachu is called Pichu , and is much smaller and weaker than Pikachu .