Layout with flexbox - flex-flow(ch. - 13)

In this exercise, in last question(mentioned below), they asked to set properties to wrap for column, but how can i check output for wrap property in column, i try to resize browser vertically but nothing happened ?

In the #column selector, set the flex-flow property to give elements a direction of column and to wrap elements. You should be able to accomplish this in one line.:

Increase the height of .box class if you want to check the wrap.