You have to go to Celadon and go to the top of the mart where there are vending machines that you can get the fresh water from.
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Buy fresh water and give it to the guard
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There is no Tea in the original Red, Blue, and Yellow games; it's unique to the Fire Red and Leaf Green remakes. Instead, you use Fresh Water, which you can buy from a vending machine on the roof of the Celadon Department Store for 200 PokéDollars.The Tea was an item in FireRed and LeafGreen to give to the 4 guards surrounding Saffron City. In Red, Blue and Yellow, a fresh water, soda pop, or lemonade will do the same thing.
By giving that girl on the highest floor of the Celadon Dept. Store that says she's thristy a drink. I think the drink is a Fresh Water, or at least it was for me.
You get it in the Veilstone Department Store on the top floor i think.