No, it’s not a riddle. It’s a question that professional soapmakers deal with all the time.
Just because something is bar-shaped and lathers up doesn’t make it “soap”.
First it has to be made of mostly oils (or fats) and some kind of alkali. When making bar soap, that alkali is sodium hydroxide. When making liquid soap, we use potassium hydroxide. But to be soap, it has to come from oils and alkali. We call that “true soap”.
Other bars may lather, but they’re sometimes made up mostly of detergents. So technically not soap. Ever notice how some of the biggest names call themselves “beauty bars” and “body bars”. That’s why. Not soap.
How can soap become something else? If any soap makes a claim like that it is “moisturizing” or “deodorizing”, it is considered to be also a cosmetic. So in that case, it’s both.
Why do soapmakers care?
Because the labeling rules are different for soap than they are for cosmetics. Professionals have to be careful that they are labeling their soaps in the right way if they are selling them to the public.
And the next time you buy some soap, take a look at the label and see if you can tell. Is it true soap? Or a cosmetic? Or both?