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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We see two dimensional projections in three dimensional space all the time – they are called shadows. Just turn on a light or stand outside, and the object created from the projection of your three dimensional body on any surface will be a 2 dimensional version of you.

Aside from projections, yes, anything that has mass in a three dimensional space must be three dimensional.

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