theory - Brim vs. Rim

While both words are use to describe the edge of cylindrical objects, they represent two different things. Brim, like on a hat, is a flat edge, where as rim, like on a Tim Hortons beverage device, has a more bulbous edge. There can only exist either a brim or a rim on a single object, never both at the same time. If a brim is rounded over, it becomes a rim, and consequently, if a rim is flattened, it becomes a brim.

It's science.