Before you are able to fly to towns you will spend a lot of time walking in "Pokemon LeafGreen." You may noticed that early on in the game, some of the guards in the guardhouses will not let you through, complaining of thirst and turning you back. While the guardhouses can usually be circumvented their is one time in particular, when you are trying to get into Saffron City, that you will need to go past a thirsty guard. Quenching the guard's thirst is pretty simple and can be done at no cost.
Travel to Celadon City. Celadon City is located to the west of Saffron City and to the north of Fuschia City.
Walk to the Pokemon Mansion located in the centre of the city and to the left of the Pokemon centre.
Enter the first floor of the Pokemon Mansion and talk to the old lady to receive the "Tea" item.
Exit Celadon City to the east to enter Route 7. Travel east through Route 7 and enter the guardhouse. The guard will take the tea and give it to all the other guards in the game. Now you will be able to pass through all of the guardhouses in "Pokemon LeafGreen" without being stopped by thirsty guards.