Eli5: How do we know that two dimensional objects are “flat”?

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Ok so I just read somebody else’s question on dimensions and that prompted me to ask this question. It is kind of hard to explain my thought process but I’ll do my best.
So we often think of 2 dimensional objects as being flat, but I feel like a truly flat object would be as un-perceivable as a 4d object to us. So if we imagine a cube made of paper we have a 3d object.
Now if we squish the cube down and flatten it we have a “2d” object, a square. But in reality that square isn’t flat because the thickness of the paper still exists. So how do we make the paper truly flat? We can cut it in half to make it thinner and flatten it out, but there is still depth. No matter how much we “flatten it” there will still be some depth. Even if it’s 0.00^ to the trillionth degree.
So my thought is for something to be truly flat it must be completely non-existent in our universe. So how can we know that it’s flat? Once we can perceive of a truly 2d object wouldn’t you also perceive an entirely new plane of existence that we can’t even fathom?

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In a three-dimensional world, there can be no truly two-dimensional material objects, because everything is necessarily made of three-dimensional objects.

So a geometric plane is *flat* only conceptually, just as a line is not infinitely thin but with a length, and a point is not infinitely small with a definite position. But in spite of that, the mathematics of planar geometry is quite useful in a three dimensional universe.

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