Volume is unstable relative to temperature.
I could take that to deep dark places, but I won’t.
It’s not a physical principle per se. It’s used in psychology to describe the idea that easily observable substances maintain the same volume if transferred from one container to another of different size or shape. It has to do with people’s ability to make deductions based on observation.
And all Newtonian physics is wrong. Nor do we have a complete grasp about what is a correct replacement. The OP’s question is however addressed by the notion of conservation of volume.
Umm, trying to be delicate here, but it’s not a physical principle, and it is a psychological hypothesis, but I’d argue with the idea of calling it a theory. I was urged to accept this as proven, but did not find the evidence. I’d call many psychological theories poor subjects for answering this OP.
Conservation of volume of a liquid isn’t a psychological theory. It is a simple theory of a physical invariant. If I have a two jars, one long and thin, which I measure to have a volume of 1 litre, and a second short fat one that also measures one litre, if I fill one with water, and then pour it into the second one, the second one will also be filled. That is a hypothesis of a physical property. The comments about psychology are directly pertinent to the OP. It is a theory that can be understood by everyone, and is so basic an idea that psychologists use understanding of the notion as a way of gauging development of cognition.
It has been pointed out that temperature changes may mean that the volume of liquid might change, but my point is that second order corrections are common, and indeed there is almost always an unstated rule with such experiments that you keep all other variables constant.
I’ve been looking around, and AFAICT there is no concept of the conservation of volume in physics. Besides the issue of changing with temperature, the is the one of the whole not equaling the the sum of the parts. This is fundamental to the the conservation of mass, momentum, and energy, but it does not hold true for volume.
Agreed, no one ever said it was a base physical law. It is however a really useful hypothesis that can be taught as a theory in it is own right, and within the constraints of a real world experiment taught to people.
It is also a very common assumption in the real world. The flow of incompressible liquids is one of the most important, and the Navier-Stokes equations become usefully simplified. For almost all real world fluid dynamics and for low velocity air flows the assumption of incompressibility works well.
I find it remains a perfect example for the OP’s question.
Color Theory is a very simple idea that kids understand:
The colors we see have fundamental properties that when mixed in various proportions to one another, can make any color possible (for adults: Within its gamut).
In other words: Red, Yellow and Blue paint, when mixed, can make all the others colors we’re capable in seeing (for adults: Within its gamut). Red+Yellow = Orange. Red+Blue = Purple. Yellow+Blue=Green. And Green+Purple=…
Another problem area. Red, yellow, and blue paints don’t behave that way. Magenta, yellow, and cyan paints do.
It is easy to demonstrate that there are people who do not understand this concept because they believe it is a physical principle, when it is not. The psychological hypothesis that this concept is intended to prove is also questionable, and not well understood. I don’t think any scientific theory will satisfy the OP unless you show that anybody can understand scientific method, and there are plenty of examples of people who cannot.
That sadly is all too true.
Germs make you sick. I think everyone gets the basics of germ theory, even if only at a very simplistic level.
Except for the people who deny it altogether, either on religious grounds or because they’re just that nuts.
If you think there’s a damn thing in the world that “everyone” can understand, then you haven’t met everyone.
I would say many spiritual myths such as demon/evil spirit concept of disease would be most universally understood.
Gravity can be universally understood by anyone with basic language skills, up to a point.
Is enough for a basic understanding of gravity. The problem comes when Ug tries to explain something outside of Ig’s personal experience, such as orbital mechanics.
Ig is going to lose interest at that point and wander off to paint a picture of a woman with enormous breasts.
I think the best answer to the OP is whatever theory that can be best demonstrated by either day-to-day experience or a simple experiment.
Ah, but there’s yet another aspect of the word “theory” that you’re forgetting - the fact that a theory must actually be based in reality and not be batshit insane.
The mainstream view of a scientific theory is that it should be falsifiable. This notion comes from Karl Popper, and was developed in reaction to what he regarded as bogus sciences of the time (Marxist political theory and Freudian psychology.) A consequence is that theories need to be predictive. A disproven theory is still a theory, it is just wrong. An experiment that attempts to falsify a theory and fails to do so validates the theory, but does not prove it.
There are plenty of crazies out there with insane ideas, a great many of these ideas won’t pass muster as a theory - it is impossible to create an experiment that can falsify them. Either because the idea is essentially non-predictive, or in the face of contrary evidence the crazies add new caveats and tweaks to the idea.
A demon/spirit theory of disease is never going to be falsifiable - mainly because you can invoke capricious behaviour on the part of the spirit/demon to explain any result you get. So it isn’t a theory. Theology perhaps, but scientific theory, no.
Of course (see my user name) But, I was aiming the theory towards kids. Kids don’t know what cyan and magenta are.
Define red, yellow and blue. In this case, blue=cyan, yellow=yellow, and red=magenta. So it’s a shift to the subtractive model of this theory.
The hypothesis is that a spectrum has elemental colors that can be combined (or subtracted) to make the rest.
The theory is that this is empirically evident, very intuitively even, buy mixing pigments or shining gels of light on top of each other… or using a prism to separate white light into the spectrum.
It’s an entire model of how our eyes/brain perceive this part of the electromagnetic spectrum. If you then want to get into the deeper, more fundamental theories of what color is, then you have far more learning and understanding to do (The electromagnetic spectrum; the visible spectrum; the emission, absorption and reflection of light; etc…)
A fundamental theory is something you learn toward, every sharpening your understanding; not something you start with, ironically.
You may not understand the inverse square law and its implications, you may not understand why there should be a force of gravity (heck! Newton himself did not understand that), but I suspect that you do know that it is a force that pulls masses toward one another, and that things fall down because they are pulled by the Earth’s gravity. To that extent, you do indeed have a theoretical (which is not to say a full theoretical) understanding of gravity.
On the other hand, I very much doubt that you subscribe to the dominant theory before Newton’s time: that things fall downward because earthy matter seeks to return to its natural, proper place at the center of the universe (or indeed, to the theory, also extant in the decades before Newton, that things fall down because they sucked downwards by an etheric vortex swirling around the Earth).