Bent
On the first panel, a Triangle-person is thinking about an equilateral triangle. They say: “You know equilateral triangle, right? All sides equal = x, all angles = 60°?”
Second panel, the person is floating in space near a star. A grid is superimposed on space, bending towards the star. The Triangle is now holding an attempt at an equilateral triangle, but at one of the corners the sides overshoot and cross. They say: “Did you know it’s an impossible shape? It does NOT close properly because our Sun bends the space around it!”
Third panel shows the Triangle standing proud, beaming and saying: “I guess that makes me an idealised representation!”
All matter bends (curves) space like this, but it’s only measurable at cosmic scales. The triangle would need to be gargantuan for us to notice: if it wrapped around our Sun like Earth’s orbit, the gap would be in the kilometre range.
Just such a thing turned up in the historical measurements of Mercury’s orbit: a gap was found when comparing to classical Newtonian models (specifically, there was an excess of orbital wobble called perihelion precession). This only got resolved when Einstein figured out the curved space bit.
Definitely not something you could have measured with a ruler during your geometry class at school.
We can’t really perceive the curving landscape directly either. It’s an intrinsic property of our 3D space, and we can’t “step out” and have a look. We can only pin down the effects of this curvature by looking at the geometric anomalies. One of those is the impossibility of an 3×60° equilateral triangle.
For completeness, this comic illustrates how space itself curves around matter, but Einstein’s theory also included time. One fun side-effect of curved spacetime is something we call gravity, but that’s perhaps for another comic.