A crumb of any food is unique when applied to the vast array of physics laws we've come up with, in that:
A crumb is still the same food as that from which it came, and is still perfectly edible (given that it hasn't undergone a drastic change of environment, like being dropped into a toilet or getting caught under someone's fingernail), and yet is virtually useless to the eater.
It's the only object that is both food and non-food.
Leaving one on your plate is the smallest possible mess you can make.
It can't be referred to as a fraction. There are no 'quarters of crumbs', only four crumbs.
One pound of, say, fruitcake crumbs paradoxically is and isn't one pound of fruitcake. You couldn't get away with selling a grocery customer a pound of cake if it were in crumb form, even though it would be a pound of the exact same substance as the equivalent weight in non-crumb form.