Is there a more compelling and easier proof of the "truth" about the inner angles of a triangle than the one I currently have:

two equal right angled triangles joined at the hypothenouses make a rectangle. Since the rectangle has all angles 90° (one fourth of a circle), that means that the non- right angles in the triangle sum up to 90. If we make the triangles symmetrical, then we get a square, at which point, we KNOW, that the angles are 1/2 of 90, ergo 45°. and since the angles work in such a way that they always have an equal sum (dunno how to prove that), that means that the inner angles are 180°.

Another explanation I see is making a super flat triangle and proving that when the maximum angle is reached, the two small ones are non-existant, and the biggest one is a line (half-circle, 180°)

Any other logical proofs with self-evident premises, without a need to measure?