The word “theory” can mean a few different things. A scientific theory is not the same thing as your uncle John’s theory about barbecue sauce.
When Uncle John says he has a theory, what he means is that he has a hypothesis — a guess that could turn into a theory someday if enough people try to prove it wrong and nobody succeeds.
Barbecue sauce is one thing, but what if Uncle John (or anyone else) has a “theory” about something less subjective, like airplanes or economics or the origins of life on Earth? More importantly, how do you know if his theory is better or worse than the scientific one? Don’t scientific theories start out as guesses, too? Isn’t Uncle John’s guess just as good as a scientist’s guess?
These are great questions.
Scientific theories do start out as guesses. But when you do a lot of science, you learn more and more about how the world works, and the knowledge makes you a better guesser. Like when you know how an airplane works and something goes wrong with it, you might be able to guess the problem on the first try, but someone like me — someone who has only a vague idea how an airplane works — could end up guessing badly for a very long time.
The other really important difference between Uncle J. and scientists is that when someone comes along and proves your uncle doesn’t know as much about barbecue sauce as he thought he did, he can go right on believing his “theory” is correct. Scientists can’t do that. Their theories have to work in real life. If a scientific theory doesn’t work, scientists keep looking until they find one that does.