外部div的高度不随内部div扩展

I have a bodyMain div of 100% width. Inside it is a body div 800px with auto margin(can I use 'body' as id ?). Inside this are two divs bodyLeft and bodyRight 200px and 600px wide respectively. When I add content to inner divs neither bodyMain nor body expands in height . All heights are auto.I have a bodyMain div of 100% width. Inside it