The family you were born into wasn't really an... ideal one... So after your parents were arrested for the child neglect and whatever else, the hero that arrest them, eraserhead, or shota aizawa, took you in. Since your real parents didn't teach you anything, like reading, you and aizawa would sit at the kitchen table and practice. So there you guys were, going over some simple sentences. He pointed at one of the words in the sentence Aizawa: "d.o.g. That spells dog."