You can also draw and cut out a bunch of differnet triangles and then "rip" off each corner and discover that they will always form a straight line (180 degree angle) when you place them side by side by side.

I proved it today in class using alternate interior angles. But this was a whole lot more visual for kids who like to actually SEE it.

That's how I've seen it done as well. It's really cool because, no matter what size or dimensions they make the triangle, when the "corners" are ripped or cut off, they will always make a straight line. The other model is very cool as well. Thanks for finding that, Alice. :thumb:

Draw a line parallel to the base of the triangle, through the other vertex. Then mark off the congruent alternate interior angles, one to the first vertex of the triangle, and another to the second. Now add the 3 angles near that top vertex-- they add up to a straight angle.