When Chopper found Robin, she wasn't reading a book. This was in and of itself unusual, but what was even stranger was that she appeared to be watching Zoro nap. It wasn't unlike Robin to observe her crewmates, but usually she did so surreptitiously while reading or talking to Nami. So Chopper, being the curious creature he was, decided to ask Robin if there were abnormal happenings afoot and if he should perhaps be worried.
"Robin?" he said, tapping her arm for attention. She glanced down at him with a smile to show that she was listening and returned to watching Zoro. "Is something wrong?"
"No, Doctor-san," she replied, mildly. "Why do you ask?"
"Well…" Chopper paused, looking from Robin to Zoro and back. "You're staring at Zoro. I wondered if he was sick or something."
"If Swordsman-san was ill, I assure you that I would have notified you as soon as I was aware," Robin said. She sprouted a row of arms from the deck that ended near another chair, which she carried back and set beside her own. "Have a seat, Doctor-san?"
Chopper watched Robin's extra arms fade in fascination before shaking himself and climbing up into the offered chair. "Thank you," he said, and Robin smiled. "So…" he tapped his hooves together nervously and glanced from them to Zoro to Robin and back to his hooves. "Why are you watching Zoro?"
"I am observing the laws of inertia," she said simply, and rested her hands atop her crossed knees. Chopper considered this.
"You mean…like how an object at rest tends to stay at rest unless acted upon by an external force?"
"Yes," Robin answered, rewarding Chopper with a smile. "But there is more to it than that."
Chopper watched Zoro the object at rest not being acted upon by an external force for a moment or two before asking "What else is there?"
"Watch," was all Robin said, and Chopper did.
It might have been only a minute or two, or longer. Chopper lost track of time as he often did in Robin's silent, appreciated company. But as he waited, Luffy bounded out of hiding with typical limitless exuberance and attacked Zoro with a surprisingly calculated pounce. Zoro, of course, snapped awake and sat bolt upright, nearly flinging Luffy overboard. Chopper giggled in spite of himself.
"Captain-san," Robin explained, "is an object that stays in motion until halted by an external force."
As if on cue, Zoro grabbed Luffy by the arms in a fit of disgruntlement and wrestled him to the ground. Luffy, though capable, made no move to escape. He appeared to be too busy laughing. Chopper found himself wondering if this made Zoro the one person Luffy couldn't move.
"It's like," Chopper began, turning in his chair to face Robin, who was giving him a patient questioning look. "It's like Luffy is an irresistible force and Zoro is an immovable object."
Robin smiled in approval. "Exactly. And what happens when an irresistible force meets an immovable object, Doctor-san?"
Chopper thought. He was fairly certain there was no answer to that question, but he was also pretty sure that Robin knew that too and was testing him. Also, where the Strawhat Crew was concerned, the laws of physics held little sway. He watched Luffy and Zoro down on the lower deck; the swordsman had the captain pinned and was still looking mildly agitated. Luffy was grinning like the fool he was and giggling in a manner that was only serving to irritate Zoro further. They appeared to be talking, but Chopper couldn't make out what they were saying. Nothing in the scene served to answer the question that still hung in the air, so Chopper looked up at Robin with his brow furrowed in confusion and said, "Nothing?"
Robin graced him with a quiet smile and nodded at their study subjects to return Chopper's attention to them. "Everything," she said simply.
Chopper looked back in time to catch Zoro and Luffy sprawled haphazardly on the deck in an array that looked somewhat painful. There was nothing unusual about that, nor about Luffy's expression of unbridled amusement or Zoro's of confused exasperation. The only strange thing about the scene was the fact that they were kissing.
Startled, Chopper looked up at Robin with his mouth open to ask whatever question decided to jump to the forefront of his boggled mind, but her calm smile silenced him as it had so many times before. It seemed that her observation was complete; she formed a chain of arms and used it to pass herself the book that lay open on the table several feet away.
Zoro and Luffy were still kissing when he looked back, in broad daylight and totally without concern. Zoro looked significantly less annoyed. Chopper considered this for a long moment before determining that really, maybe it wasn't all that unusual after all.