level2
responsive design
rows:
columns:
we can use multiple columns:
we can use .col-md-*
we can do something like this
Adding row, make code easier to read
popular blogs with siding bar
leave one column as padding
Empty column
much better way to leave padding
实现不同size的设备上的不同显示：
遵循从小到大的继承
remain padding for book today!
what if we only want the image to show on medium or large resolution?
we use .hidden-*
what is we want to show some image?
