Get the tea from the tall building in celadon city, first floor, old lady
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At Celedon City, go talk to the old lady in Celedon Mansion. She'll give you some hot tea and that's what you give to the guards.
got to the mega mart by 5 drinks on the top floor in the machine then go to them and they will talk and then they will have the drink and now you can walk through Edit: Or just talk to the old lady in the Pokemon Mansion in Celdon she will give a hot tea drink.
In celadon city at the top floor in the big mall there is a vending machine, give it to a guard and he will share the drink with all the other guards so you can go through these guards now
Give the guards a drink. No, seriously. You use the underground passages to get to Celadon then go to the roof of the department store and buy a drink off the vending machine. Doesn't matter which one. Then you give it to any of the thirsty guards and they'll share it with their fellows and then you should be able to get through all of the closed roads.
In Celadon City you need to go into the Celadon Mansion (Front Door) and talk to the little old lady babysitting the Pokemon. She will give you the tea.