TIL The ancient Romans created an artificial sweetener called "Sugar of Lead" by corroding lead in vinegar. It has a mildly sweet taste and also causes brain damage

When lead combines with acid it creates lead acetate which interacts with the sugar receptors in the mouth. I'm not sure why, I think the molecules just happen to have a similar shape.

EDIT: after looking at the molecule for lead acetate, it appears to resemble two simple carbohydrates connected by an atom of lead, so I'd imagine the receptor sees the outside and assumes it's something sweet.