How to show the angles in a triangle add up to 180 degrees

This video can not be played

To play this video you need to enable JavaScript in your browser.

A short video on how to show the angles in a triangle add up to 180°.

Image caption,
Click to see a step-by-step slideshow.
Image caption,
WHAT YOU NEED: piece of card/paper, ruler, scissors and pen.
Image caption,
STEP 1: Use the ruler to draw a straight horizontal line, draw a diagonal line from the end of the horizontal line...
Image caption,
...and draw another diagonal line to join the points and mark the angles inside the triangle. Use the scissors to cut the triangle out.
Image caption,
STEP 2: On another piece of paper, use the ruler to draw another horizontal line.
Image caption,
STEP 3: Tear each corner off the triangle.
Image caption,
STEP 4: Place two of the triangles along the horizontal line. Turn the third triangle upside down and slot it between the other two on the line. It should fit perfectly.
Image caption,
The angles in the triangle you drew all add up to 180°. Remember, all angles on a straight line add up to 180°.