Issue each student a piece of blank, white printer paper, a pencil and a protractor. Challenge students to answer the question, "What two angle measures create a straight line or 180 degrees?"
Allow the class five minutes to figure out the problem and ask a few students to share their answers with the class. Instruct each student to turn his paper over and use the protractor's bottom edge to draw a straight line. Ask him to measure that straight line, ensuring that it is 180 degrees.
Instruct each student to line the protractor's bottom edge with the drawn line and make a tick mark anywhere on the curved edge of the math tool and on the straight edge at the center or 90-degree mark. Connect the tick mark with the straight line's center mark using the straight edge.
Challenge the students to measure each angle created by the drawn connecting line and write the measurement in the angle. Add the two measurements to reach 180 degrees, or the measurement of a straight line.
Compare the angles to a situation the children may be familiar with. Create a scenario that requires them to add two angles to 180 degrees such as a trick turn on a bike or a car slipping on the ice. Ask each how many more degrees must the object turn in order to face the opposite direction (180-degree turn). The number of degrees the object has already turned added to the number needed to turn to the opposite direction reinforces adding two angles to a straight line or 180 degrees.