Sango managed to keep her mouth shut when Inuyasha came to join her and Toutousai, not by walking down the path but by floating down the stream. She had no idea how such a thing had come to pass, but she did not wish to aggravate him further, so she decided not to ask no matter how curious she was.
"Listen up, Toutousai," Inuyasha began, "this sword is a major pain in the ass. Can't you make it lighter again?"
"I'm afraid not. The sword cannot be returned to its original state. You'll just have to get used to it."
The hanyou kept grumbling as he wrung out his clothing; being soaked was obviously doing nothing to improve his disposition. "Sorry, old man. That doesn't work for me. You wanna try again?"
"You've been through a lot lately," Toutousai observed, sounding utterly unfazed by Inuyasha's ill temper. Of course, when one was as powerful as Toutousai, it was probably easy to take Inuyasha lightly. "First becoming a human and then becoming a monster."
"Don't call me a monster!" Inuyasha snapped. Obviously this was a sore spot for him. "Even I don't understand why it happens."
"You don't understand," Toutousai echoed. "You're an idiot, you know that?"
Sango did not think he was being entirely fair to Inuyasha, but she could see his point. Even before Myouga explained what was going on, she had begun to have suspicions about when and why Inuyasha started to transform into a full youkai. In hindsight, it wasn't very hard to guess. It also wasn't hard to guess that Toutousai, who had made it in the first place, knew exactly what purpose that sword served. He probably even knew why. That would explain his reluctance to speak straightforwardly.
By this time Miroku and Kagome had come over to join the rest of the group. They waited as avidly as Sango for Toutousai to keep going.
"Myouga told me that when it looks like you're going to die you transform into a full youkai." He paused. "Here is what I think, though: I think transforming into a youkai is not Inuyasha's greatest strength."
The hanyou in question did not look like he much liked the way this was going, but he finally had the sense to be quiet and listen instead of loudly protesting.
"Inuyasha, as I've said too many times already, the sword is heavy because of the weight of your fang," Toutousai explained with far more patience than Sango would have been able to muster at this point. "As it was, the Tessaiga was made from the fang of your father. Without realizing it, you were relying on his power to protect you. Now, however… the Tessaiga is no longer made from only your father's fang. You must now wield your own fang and protect yourself, and to do that you must become truly strong."
Sango had not considered it in quite that light before. Toutousai made it sound very impressive, so perhaps that would be the encouragement Inuyasha needed to pursue it seriously.
"It's just as you say, Toutousai," Miroku chimed in.
"You're already strong, Inuyasha," Kagome added, smiling as she tried to cheer him on. "I'm sure you'll become strong enough to wield the new Tessaiga in no time!"
"Well, yeah…" he said, looking suddenly uncomfortable. It wasn't hard to figure out that Miroku and Kagome were trying to lead him to the answer they wanted. "Probably."