Give the guard a drink of water. You can get one from the vending machine in the Pokemon Mart in Celadon City. The vending machine is on the top floor. Once you buy one from it, walk past him and he'll ask you if he can have your drink. Say yes and you will be able to pass.
Buy fresh water and give it to the guard
Lemonade
go to the house with the old lady and talk to her and you will get a cup of tea then talk to the guard
a lady in celadon give you a cup of hot tea. she is in one of the tall buildings near the Pokemon center
Give the guard a drink of water. You can get one from the vending machine in the Pokemon Mart in Celadon City. The vending machine is on the top floor. Once you buy one from it, walk past him and he'll ask you if he can have your drink. Say yes and you will be able to pass.
You'll need to make it to Cealdon City first and buy a drink from the vending machines in the department store. Head east out of the city and give the guard the drink and he will let you into Saffron.
Buy fresh water and give it to the guard
Lemonade
go to the house with the old lady and talk to her and you will get a cup of tea then talk to the guard
i don't know Pokemon yellow for gameboy be cause i don't have that game. i know that the star Pokemon is pikachu using thunderbolt
Top of celadon department store in the vending machines
a lady in celadon give you a cup of hot tea. she is in one of the tall buildings near the Pokemon center
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.
Go to Celadon City and buy a drink for the guard and he will let you through
go to the Celadon Mansion and talk to the old lady. she'll give you tea. give this to a thirst guard and he'll share with the others. no prob!
I think you must be talking about the guard who checks that you have all 8 kanto gym badges when entering the indigo plateau. If you don't have them, you can't pass.