The Sum of the Interior Angles of a Triangle Equals 180 Degrees by Paper Folding
In high school geometry, various manipulatives (in class demonstrations, etc.) are used to show that the sum of the interior angles of a triangle equals 180 degrees. One simple example is to have the students cut off the three angles of a paper triangle and then rearrange these so that they all share a common vertex. The result is a visual proof that the sum is 180 degrees.
This Demonstration uses paper folding to show the result.