Korin Tower
Sacred Land of Korin · The climb is part of the lesson
Korin Tower rises so far above the Sacred Land that climbing it bare-handed is itself a test of endurance. Goku attempts it because Mercenary Tao has shown him, unmistakably, that his current level is not enough.
Reaching the top does not end the training. It only gets him into the room with the teacher.
Why it matters
Dragon Ball turns geography into training. Before gravity rooms and chambers outside normal time, improvement can begin with something as simple and miserable as climbing.
Sacred Water (Seizui)
Korin's fake shortcut
The Sacred Water is supposed to grant enormous strength to whoever drinks it. Korin even makes Goku take it from him first.
The trick is that the water is ordinary. Goku becomes stronger because he spends days chasing someone faster and more perceptive than he is, repeatedly climbing, reacting, failing and trying again.
Why it matters
It is one of the cleanest statements of Dragon Ball's training philosophy: the prize does not make Goku stronger. The work required to earn it does.
Ultra Divine Water (Choushinsei)
SPOILER!
King Piccolo crisis · A real and dangerous gamble
The Ultra Divine Water is the opposite of Korin's earlier trick. This time the substance really can draw out hidden power, but it is poisonous enough that drinking it is a genuine survival gamble.
Goku accepts because King Piccolo has pushed the situation beyond ordinary training time. There is no tournament date months away and no patient rematch waiting for him.
What changed
Korin's first lesson rewards work. The second offers a desperate shortcut precisely because the story has become desperate.
Senzu Beans
Korin's restorative bean · Small supply, enormous value
Senzu Beans begin as one of Korin's odd little resources and gradually become indispensable. A bean can restore a badly depleted fighter and provide enough nourishment to keep someone going for days.
What makes them useful dramatically is scarcity. Korin does not operate an infinite vending machine, and later battles repeatedly turn the remaining supply into a tactical decision.
Why it matters
One of Dragon Ball's most important recovery mechanics starts here as something a strange cat happens to grow at the top of a tower.