go to the safari zone in fuchisa city and find the warden's gold teeth and give them to the warden
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If you mean strength you can find that in iron island talk to the guy with the blue hat and he will give you strength if you mean something else i don't know
you have to find it
In the Pokemon forest
You can find the HM strength at the top of the lost tower south of solaceon town :)
You have to find the (hm) strength make a pokemon learn it then when you click on a rock you will be able to push it I dont know where to find strength as I have never played leaf green but I know how to use strength