Once you battled Sudowoodo (by using the Squirtbottle to attack you), go east to a large man on Route 36.
He'll congratulate you because you cleared the path and give you Rock Smash.
It is a relatively weak move with 20 attack points. I recommend giving it to a fighting Pokemon or a Pokemon who has a learnset that has a lot of offensive moves to spare like quilava.
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After you battle Sudowoodo, the man near the ruins of alpha entrance will thank you and give you rock smash.
in Pokemon silver, gold, crystal, yellow, blue or red rock smash don't exist. in the new versions rock smash is a HM.
rock smash isn't in Pokemon red version
Go to where you caught/battled Sudowoodo, and to the right, there's a fatty who gives it to you!
A wide variety of Pokemon can use Rock Smash. Most Fighting, Rock and Ground type Pokemon can learn the move Rock Smash from the TM or HM depending on the Pokemon version.