¤ Go to Viridian City and talk to the coffee guy who shows you how to catch a Pokémon. He will ask you if you are in a hurry? Say NO! then he will show you how to catch a Pokémon (WEEDLE). ¤ Now use your flying Pokémon and FLY to Cinnabar Island. ¤ Go to the right side of the island and SURF. Keep going up and down on the right side of the island's shore. -> Now Pokémon will appear, just Run until you encounter a Pokémon which looks like a fuzzed graphic. Sometimes it'll be called MISSINGNO. but sometimes it will not have a name, it will just be blurred. (Note: Do not catch this Pokémon 'cause it can wipe your saved game!). ¤ Fight the MISSINGNO. or you can just simply Run away from it. -> Once you have done this, whatever item you have on the no. 6 slot of your item list will now be multiplied by a considerable amount. This will be good for RARE CANDY, MASTER BALL or any item you want!
