介绍3D图形-square pyramid

用纸做一个正方棱锥，介绍一下 展开图和面

可以发现生活中的正方棱锥，说一说

例如：

This is

square pyramid.

This is net.

One, two, three,

four, five.

There are five faces.

Four faces

are triangles.

One face is a square.