Tanjiro and his wooden box make the perfect traveling companions for Nezuko. But how does Nezuko fit inside the box?
Nezuko is able to fit in the box because her demon powers let her change size. Her height and strength shifts depending on how much power is used. Once Nezuko is done fighting, she can fit in the box because of her small resting size.
She can also change her height in order to protect herself. This skill came in handy when Kanao was ordered to hunt Nezuko down but couldn’t land a hit. Nezuko’s bamboo muzzle also keeps her safe from acting out as a demon.
The Plot Behind How Nezuko Can Fit In The Box
Nezuko first changed shape in chapter #1 when she attacked Tanjiro. Her height and muscles grew in accordance with her strength, and she did not deflate back down until being subdued by Giyu.
Tanjiro and Nezuko then ventured to Urokodaki Sensei’s home by Giyu’s request. On their travels, Tanjiro constructed a makeshift basket for Nezuko to sleep within.
“Nezuko. You made yourself turn into the size of a grown woman, right? Can you do the opposite of that and shrink?”
Tanjiro to Nezuko | Chapter #2
It wasn’t until the duo reached Urokodaki that they received a more formal box for Nezuko. The sensei constructed the new box after Tanjiro passed the Final Selection and had been assigned on his first demon slaying mission.
“This was the box I carried my sister in during the day. It is made with an extremely light type of wood called “Mist Cloud Fir.” After painting it with “Rock Paint,” the outer layer hardens, strengthening its durability.”
Tanjiro explaining the box’s upgrades | Chapter #10
The reason Nezuko sleeps in a box is to protect her from sunlight. She’d burn the same as any other demon, although she can stand outside during the day so long as it’s very cloudy.
So that’s it. Nezuko can fit in the box by changing the shape of her body at will. The size usually corresponds to how much strength she needs or previously used. She can also change shape in order to protect herself if need be.