Background goes bye-bye

Hola

I’ve div tagged my header, sidebar, footer and post entries as different tags. They are all inside a wrapper div, with white as background color.

When I use the float function on any of the divs inside the wrapper, the background doesn’t affect it anymore. Actually, it ignores the wrapper background and instead shows the real background behind that particular div.

Why’s this happening? And is there a way around it?

Thanks
-Fred