Missing angles in a triangle leads us to the angle sum theorem which states that there are 180˚ in every triangle. Really??? In EVERY triangle? How is this possible?? There are a lot of theorems in geometry that we TRY to prove to students, but perhaps it's difficult for everyone to make the connection. The angle sum theorem in triangles though is a theorem that has a low floor and should allow more students to really internalize the idea.
Here are a few ideas for "proving" to students that there are 180˚ in the interior angles of EVERY triangle.