POV Garp
Garp observed Luffys fight with the Tiger King through Observation Haki, what he found is both good and bad, good because the brat fought much in the same way Garp did, like a brawler, using a few tried and tested moves as a base and applying them creatively as situation demanded, bad because the brat is too reliant on his Devil Fruit.
Garp always found Devil Fruits to be an useless crutch, oh they are a major power boost and when used with creativity they can be an excellent force multiplier, yet the advantages ended there. The inability to swim is not even the worst of it, Devil Fruits when consumed they become too intrinsic to the consumers entire being, users become useless lead weights even in a bath tub much less submerged in the Sea, Seastone saps everything from a Devil Fruit user, their Devil Fruit granted powers, their physical strength, their Haki and even their mind becomes foggy, and the adverse effects persists for even awhile after Seastone is removed from their person. At least the brat did not seek one out to eat it, instead eating it by accident, another reason to beat the shit outta that Red Haired Brat. In Garps own mind the Worlds Strongest are strong not because of their Devil Fruit Powers but despite of them, even being so strong, all Garp needs to take them down is to submerge them in a tub of sea water and their own bodies will betray them by drowning them.
Whats done is done, no use crying over spilled milk, the brat will have to find trusted people to watch his back and cover his weaknesses on his own. What Garp could do is to make Luffy strong enough that he doesn't end up in a situation where his obvious weakness could be exploited, too often. He is Garps grandson after all, almost dying due to sheer carelessness is a family tradition.
As Luffy was about to land a killing blow on the Tiger King, Garp appeared behind the brat in a flash using sheer physical speed and hammered his patented Fist of Love down on the brats head with a shouted, "Luffy!"
The brat clutched the instantly appearing lemon sized bump on his head shouted with a quiver and obvious note of fear in his voice, "Gramps", and instantly tried to leg it. But Grap already has a iron grip on the brats neck, so all he managed to do is stretch his body and head to the other side of clearing before snapping back to Garp.
The Tiger King neglected by the family reunion took one look at Garp and got outta there so fast, it appeared to have succeeded in learning and using a form of Soru in its sheer desire to run away.
Good, the cat will serve as a better sparring partner for Luffy later on.
"Call me Grandpa, you brat. Now give your loving Grandpa a hug", Garp demanded.
For all his grumbling Luffy did give Garp a hug, the brat has always been that way, Luffys love has always been all encompassing and without reservation for the people he chose to love.
"Brat, you are WEAK!", Garp declared bombastically.
"Gramps, No! My punch is stronger than a pistol", Luffy rejoined confidently, thrusting his fist before him.
"I can catch bullets with my teeth and eat them for breakfast, brat. You are weak", Garp asserted with complete sincerity.
"Then I will grow stronger, I will become the Pirate King", Luffy replied with conviction. Garp already raised his fist and Luffy no doubt expecting another Fist of Love, closed his eyes and cowered. Yet Garp broke the routine and only flicked Luffys nose as gently as he could manage, prompting Luffy to open his surprise widened eyes and stare at Garp.
"Why?", Garp asked intently, for the first time.
"Because I want to be FREEST person in the World", Luffy proclaimed with a blinding grin.
"To be Freest person in the world means to be Strongest person in the world Luffy. To be Stronger than ME", Garp said.
"Then I will become stronger than you", Luffy replied still with that blinding grin with utter confidence.
"Will you now? Come, I want to show you something", and started walking deeper into the forest.
The grandfather and grandson duo reached their destination, which is the top of Mt. Columbo, which is somewhat flat and has an enormous boulder of fifty feet height and eighty feet wide.
And Garp promptly hammered down his fist on Luffys head without a word, who got pancaked to the floor from top down, before snapping back to his normal shape in half a second, "Now, tell me brat, did that hurt?"
Luffy who looked at Garp in wonder, tentatively answered, "No?"
"And why didn't it hurt?", Garp asked.
It took Luffy a full minute of thinking before he answered unsurely, "Because I am Rubber?"
Garp nodded signaling his agreement, "Because you are essentially Rubber, since you ate your Devil Fruit, blunt force doesn't really work on you. Now tell me does this hurt?", so saying Garp flicked Luffys nose.
Who immediately wailed out, "OW!", and made a muffled reply from hands that clutched his nose obstructing his mouth, "Yes, it does!"
"Why?", Garp asked. This time it took Luffy five minutes of thinking and smoke literally pouring out of his ears to arrive at an answer, "You have a Mystery Power that lets you hurt me even if I am rubber!"
Garp nodded and said, "Good enough! Its called Haki. Everyone has different definitions for it, 'Presence', 'Fighting Spirit', 'Intimidation', 'The act of not Doubting', some have complicated explanations for it, some can explain it in simple terms", seeing Luffy getting a glazed look in his eyes, Garp flicked Luffys forehead with a haki infused finger before continuing, "but for me it means, Bending the Worlds rules with my Willpower. Like being able to hurt a rubber brat with blunt force, even though by the rules of the world it shouldn't be possible to do so. Do you understand me?"
Luffy bent his rubber neck complete ninety degrees to the side and said slowly, "Noo?!"
Garp sighed out and massaged his temples and said, "Its a mystery power that lets you do things you are not supposed to be able to do if you want to do it strongly enough, Luffy. Do you understand now?"
Luffy nodded his head enthusiastically and said, "Its a mystery power that lets me do whatever I want."
Garp dragged his hand down his face and said brightly, " Good enough for today. NOW! Onwards to training."
