After defeating Brock and earning your first Gym Badge, head east from Pewter City. You'll pass through Route 3, encountering more trainers and wild Pokémon. Mount Moon awaits, a cave filled with Zubat, Geodude, and the elusive Clefairy. Navigate its winding paths, collect fossils, and defeat Team Rocket grunts. Emerge from Mount Moon onto Route 4, and soon you'll reach the beautiful Cerulean City, home to Misty's Water-type Gym.
