floating div's container height reduces to zero

Hello everyone,

My problem is that I have a parent div with two floating divs inside.I want parent divs hight to adjust according to the container div's height.but i have noticed that the container divs height reduces to zero while the floating divs are displayed as required .This causes a problem when i am trying to make a responsive website as the height and width of child are not getting adjusted with resolution change

I have tried the following things but without proper result
1)set parent's height and width to 100%
2)set parent's overflow:hidden
3)clearing floats of the parent

please help as I cannot understand the reason behind this

Thanks and Regards
Ranju