My dad’s passion since he was a child was horses. He lived and breathed them and after he got married he (with my mother’s significant help!) was finally able to afford a house with a bit of land and he bought his first horse that he broke and reared to become a champion cross country girl before living out her final days with him and dying aged 23. (He had others along the way too) Talking to him about anything horse related was like opening an encyclopaedia. He wasn’t an educated man but when it came to horses I doubt there was a book printed that he didn’t own. He could talk all four legs off any horse he encountered.
Ive come across MNers who are extremely knowledgeable in various things, like Latin, or World War II, or ballet, or DIY, or dogs. It fascinates me.
I, however, know nothing. Sometimes I forget which way to turn the key in the front door 
I am absolutely crap at retaining information. I have studied and achieved brilliant grades in exams. But ask me about the topic a week later and you’ll get a blank stare.
So, fascinate me all you interesting people! What’s your thing?