"I move...":

How large a back counts? If you take a stool (3 tall legs) and put a very, very low back on it just 1 inch tall, is it a chair? What if the back is 2 inches? 3 inches? 2 feet? At what point does it become a chair?

Or take a typical dining room chair. Cut off the legs and rearrange them so that it only has 3 legs. Is it a stool?

This is a difficult problem that has been debated by philosophers for thousands of years. If anybody writing comments to this blog has an answer, they are guaranteed to become as famous as Aristotle (and perhaps win a Nobel prize. Yes, they don't give Novel prizes for philosophy officially, but sometimes they cheat, cf Bertrand Russel).

Please try my web-based experiments