Pierdan believed this Invention Mouse was special mainly because... it gained self-awareness shortly after birth.
"Probably on the third day after it was born, it showed signs of melancholy for the first time."
Melancholy is an emotional state, and emotions are the starting point of all life forms' perceptions of the outside world. The more complex the emotion, the deeper the perception it represents. Melancholy is evidently a more complex emotion.
"Afterwards, it took just one month to overcome the most difficult vocalization hurdles for mice and learned to express meaning with throaty sounds."
"Next, it spent three months learning the Pirush language."
"After mastering the language, it gave itself a name."
As Pierdan spoke, he glanced at Pixi and then said softly, "It named itself... Nak Su."
"Nak Su?" Pixi's expression froze upon hearing the name, and he murmured, "Why does it sound familiar?"
