[jasim]

Imagine you take a piece of paper, draw 4 vertical and horizontal lines dividing the page into 16 cells. Now when you place an element in any of these cells with width: 100%, they'll take exactly the width of the grid cell. It can't grow beyond that.

You have a lot of flexibility in choosing the width and height of each cell, but they are rigid.

In case of Flexbox, we don't specify any fixed lines to which the elements will size. Instead, we say general rules saying elements can grow to as much size is available, or stay at the beginning of the row etc. The layout is determined to a large extend by the elements we place rather than on a fixed set of gridlines.