A lot of people postulate that you can't fit your head into a cereal box, but I tried it, and you can't. That's called proof.
Need more proof? You cannot walk on hot coals. Your feet will hurt really bad and you'll have to jump off. I've tried it - not just a theory anymore.
If you try to put your head in a ceral box, it may make it, but not while maintaining the structural integrity of the box. Depends on the box. That's called a variable.
Need more proof? A grown man can no longer fit into a kitchen cabinet. He may think he can because he remembers doing it when he was a kid, but those cabinets are smaller now because you're bigger. That's not a theory, I've tried it!
Variable: he may be a very small man in stature. He may also be small in character, like, petty. But that's a different variable.
And all this stuff - I didn't try it just once. To be scientifically proven it has to be repeatable, so I did many, many trials over hours of field research. You certainly cannot fit your head in a full cereal box. Full means still containing all the cereal.
You cannot headbutt through a door. My door stands resolutely as proof to that, mocking me as we speak. You win this time, door!
I hate this weak human forehead. This isn't over, door!