如何将inner-div固定在outer-div的底部，以便在outer-div扩展时移动

So I have a inner-div inside an outer-div. I want the inner div to function as a footer, so it has to stick to the bottom of the outer-div. When the outer-div expands (in height), I want the footer to move along with the bottom of the outer-div. I hope you understand what I mean?So I have a inner-div inside an outer-div. I wa