"Obviously, Ash Ketchum couldn't stay home to become a Pokemon Master. His last name is freakin' Ketchum! He had to go out into the world, and along the way, he might've acquired some useless Pokemon. He had a Metapod at some point in time. He turned into Butterfree eventually, but Metapod was a worthless Pokemon. All he did was sit there and harden. So while he may have been sitting there and hardening, he was collecting experience in order to become a Butterfree."