Ep. 15: Episode 15
Mio and Yuuko attempt to prove that Nano is a robot only to fail, but Mai manages to film evidence. Nano tries to convince the Professor to remove the wind-up key on her back as it's hindering her school life, but gets mad and shoots the Professor with a pellet gun every time a dubious item comes out of her body. Ms. Nakamura plans to kidnap Nano, as she is interested about her robotic nature. When her drugged coffee fails to work on Nano, she decides to use a taser, only to mistakenly use it on Tsuyoshi. Mr. Takasaki becomes conflicted upon seeing Ms. Izumi and Makoto being close, unaware that they are siblings. Nano asks the Professor about the wind-up key again, but much to her chagrin, the Professor has installed a reaction feature that causes Nano to react in insane ways every time the Professor makes a joke. (Source: Wikipedia)