Back of My Head (後頭部, Kōtōbu) is an extra chapter included with Volume 25.
Summary[]
About one and a half years ago, in a park, Saitama was seen doing push-ups during his training regiment. After finishing his work-out, he took a shower and realised that he was constantly losing hair. As he got a feeling his scalp was sticking out more than usual, he wondered what was happening to the back of his head. He wanted to know but there was only one mirror for him there. Afterwards, Saitama tried turning his head as quickly as possible in an attempt to see the back of his head that was reflected in the mirror. Despite having failed many times at first, in the end, Saitama was apparently able to catch sight of the back of his head.