1、先来画一个正方形：
连结四条线段。
Graphics3D[
Line[{{1, 0, 0}, {0, 1, 0}, {-1, 0, 0}, {0, -1, 0}, {1, 0, 0}}]]
2、把线段变成管。
Graphics3D[
Tube[{{1, 0, 0}, {0, 1, 0}, {-1, 0, 0}, {0, -1, 0}, {1, 0, 0}}]]
3、或者，还可以写为：
Graphics3D[
Line[{{1, 0, 0}, {0, 1, 0}, {-1, 0, 0}, {0, -1, 0}, {1, 0, 0}}]] /. Line[pts_] -> Tube[pts]
4、限定管子的粗细程度：
Graphics3D[
Line[{{1, 0, 0}, {0, 1, 0}, {-1, 0, 0}, {0, -1, 0}, {1, 0, 0}}]] /. Line[pts_] -> Tube[pts,0.1]
5、还可以这样：
Graphics3D[
Tube[{{1, 0, 0}, {0, 1, 0}, {-1, 0, 0}, {0, -1, 0}, {1, 0, 0}},0.3]]
6、改变拐角处的接口样式：
Graphics3D[{JoinForm["Miter"],
Tube[{{1, 0, 0}, {0, 1, 0}, {-1, 0, 0}, {0, -1, 0}, {1, 0, 0}}, 0.1]}]
7、改变颜色，实现渐变色：
Graphics3D[{JoinForm["Miter"],
Tube[{{1, 0, 0}, {0, 1, 0}, {-1, 0, 0}, {0, -1, 0}, {1, 0, 0}}, 0.1,
VertexColors -> {Red, Green, Pink, Blue, Red}]}]
