Watching Hocus Pocus through a child's eyes, you probably think Winifred Sanderson is the only one of the Sanderson sisters who has any brains at all. That's because that's what Winifred wants you (and her sisters) to think — more on that later. In fact, it's the "bumbling" Mary Sanderson that's actually the smartest of the whole group, and possibly the most powerful.
Let's start with the fact that Mary Sanderson has a highly attuned sense of smell when it comes to kids. She's basically the sommelier of children. She can tell you, just by smell, if there is a child in the room, whether that child is a boy or a girl, and their age (down to about six months). Don't believe me? In 1693, she could smell Thackery Binx in addition to his sister Emily in the Sanderson house. "I knew I smelled a boy!" she exclaimed when he was discovered. Then, in 1993, she was able to sniff out Dani, asserting she could smell a little girl, around 7-and-a-half or 8 years old.
But it isn't just her sense of smell that proves she's smart and powerful, she also knows about futuristic things she has no business knowing about. Take for instance when she is smelling for the children and detects scrod instead, which she notes is sometimes cooked with a little bit of bread crumbs and margarine, maybe a bit of olive oil. Clearly Mary has powers beyond even her own understanding.