我们将用一系列这样的盒子来学习css的用法。盒子套盒子，很有想法很有创意。
we owned these boxes to show some interesting things. Join me soon.
this is a little box inside box2, which to float on the left or right. what's the look like?
当我们应用float属性的时候，盒子浮动在它流位置上，但是它原有的位置被下方的流盒子填充了。