Why does wrapping div not expand around floating elements?

I have 开发者_如何学JAVAtwo columns in a center column. (They are all div tags.) When I set the inner divs to float:left, the outer div does not wrap around the inner divs.

Why, and how do I fix that?

set overflow: auto on the outer div.

Why does setting overflow alter layout of child elements?

set overflow:visible for that div, or remove overflow property.