According to the pixiv comment by the artist, it's a joke about how Mikoto comes up with the idea that since he was impervious to her electrical attacks, he must be made of rubber.
varrach said: According to the pixiv comment by the artist, it's a joke about how Mikoto comes up with the idea that since he was impervious to her electrical attacks, he must be made of rubber.
So she was reading Luffy vs Enel then?
Ve~cto~rs~*grab**whoosh**pull*Why won't it stretch?!As if it could!!!!The End