You need to get around to Celadon (via Lavender Town), visit the Pokemon Mansion there and an old lady will give you a hot drink, take it to the guard and he will pass it round.
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first you go to the town to the right where the grass gyme is and ask the old lady in one of the buildings near the pokemon center then she will give you tea and you take the tea to the guard
At the top of Celdon city , well there is a building there and right at the last floor you can go to some vending machines and buy water and give it to them
He is the sleeping pokemon blocking the entrance to cycling road and past vermillion city
You must 1st beat the game by taking on the Elite Four. Then if you get the ruby and Sapphire so that you can trade with Hoen region (Pokemon Emerald,Saphire,Ruby) then go talk to the guard and you will be able to get in. In cerulean cave you can find Mewtwo if you go the write way and be sure to bring escape ropes.
If you mean the guard by the elavator, you can't get passed him.