Prove that the sum of the interior angles of a triangle is 180 degrees. Do you remember that a straight angle = 180 degrees? You have shown the sum of the angles of a triangle = 180 degrees.

A straight angle is an angle that measures exactly $180^\circ$. It forms a straight line.

The activity described (drawing a triangle, cutting out its angles, and joining them along a straight line) demonstrates that when the three interior angles of any triangle are placed adjacent to each other, they form a straight line.

Since the combined angles form a straight line, their sum must be equal to the measure of a straight angle. Therefore, the sum of the interior angles of a triangle is \boxed{\text{}180^\circ}}.