Cut a triangle out of paper.
先用纸剪一个三角形。
Tear off its three corners.
再把它的三个角都撕下来。
Put the three pieces side by side.
把这三块角，边对边拼在一起。
They make one straight flat line.
哎，正好拼成一条平平的直线。
A straight line is 180 degrees.
一条直线，就是180度。
So the three angles always add up to 180.
所以三角形的三个角，加起来永远是180度。
