How do you move boulders on Pokemon in Pokemon LeafGreen?

To move boulders on Leafgreen you must first get the Strength HM. To get the Strength HM you have to go to the Safari Zone in Fuschia City, and go deep into the Safari Zone until you find the Secret House(you can go there to get the Surf HM) look around near the Secret House and you will find an Item called "Gold Teeth" those are the Safari Warden's lost teeth, now exit the Safari Zone then go to the Warden's House(it is one of the houses in Fuschia) talk to the Warden and he will give you the Strength HM in exchange for his missing teeth.