Going through Route 9 is necessary to reach Route 10, but it can only be accessed by having a Pokémon that knows Cut. In addition, make sure to pick pick up Flash from Route 2 before going into the Rock tunnel, 'cause it's going to be dark.
This route is East of Cerulean City, and contains the entrance to the Rock Tunnel. Going through Rock Tunnel may seem like a roundabout route, but it's the only way, as there is a Snorlax blocking Route 12 and the thirsty officer blocking Saffron City will not let the player in.
Professor Oak's Aide
Inside the Pokécenter you will find one of Oak's Aides. Talk to him and, if you have complete data on 20 types of Pokémon, he will reward you with an Everstone which can be held by a Pokémon to prevent it from evolving.
If you missed catching a Pikachu while you were in the Viridian Forest, this is the only Electric pokemon you'll see until you can visit the Power Plant with surf. Voltorb's only offensive Electric move is spark at level 21, which while good can't compare to Thunderbolt, which Pikachu gets at 26. It's a good idea to pick up the TM from Rocket Game Corner in Celadon City if you plan on making Voltorb your primary Electric type. If you already have a Pikachu you can skip this Pokemon.
Voltorb
Spearow
Ekans
Sandshrew
Heidi
Pikachu 20
Clefairy 20
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