I enjoy the problems posted on the Five Triangles blog. It says they're appropriate for grades 6 to 8, but they are often hard enough to make me sweat a bit. This particular problem demanded to be solved, and made me sweat more than most.
When I finally got a chance to look it over carefully, my first response
was bewilderment. Then I had a few ideas, got out paper and pen, and
drew some lines in. I was stuck again, but when I stated my new simpler
problem carefully, and drew a new picture for it, I got it. To check, I
used geogebra to construct it with just points, lines, and circles.
Playing with these problems makes me want to teach a geometry course.