Mary knows literally nothing about society or how to function as a human being. She doesn't know how to use things like the shower or the stove, she doesn't know what telephones are, or the towns church bells startle her.
But, thanks to Spencer, she learned a few things about herself. like, that she likes pop, specifically from the 80s (surely once Spencer was listening to Girls Just Want To Have Fun and Mary started listening to it on repeat for hours every now and then.) Or that she likes yogurt, since that was the first thing she ate when they found her (it's an obsession at this point). And speaking of obsessions, Mary doesn't know what TV is or how it works, but she spends a lot of time glued to the screen. Spencer had to teach her that it's not healthy for her eyesight, lol.
And by the way, I changed a little bit of the lore. Spencer doesn't have his own house; he's looking after his mother's house while she's on vacation.
