Angle Relations in Triangles and Quadrilaterals

How do I know that every triangle has three angles that add up to 180°?
Well, try this. Draw any triangle you like, using a ruler to make sure
you have three straight lines. Then, cut out the shape, rip off each corner
and arrange them with their points together. They should all fit together
on a straight line! 

You can do the same thing to prove that all quadilaterals have four
angles that add up to 360°. Draw any quadrilateral you like, using
a ruler to make sure you have four straight lines. Then, cut out the shape,
rip off each corner and arrange them with their points together. This time
they should all fit together, forming a full circle! 
