What is the simplest jQuery way to have a "position:fixed" (always at top) div?

I'm relatively new to jQuery, but so far what I've seen I like. What I want is for a div (or any element) to be across the top of the page as if "position: fixed" worked in every browser.

jQuery Masonry from bottom up

Does anyone know how to make jQuery masonry stack from the bottom up? I wrote some rudimentary JS to stack things from bottom up but it couldn't do masonryish stuff like stacking the next brick on the shortest column and bricks that span multiple columns. Since I'm not good with Math, looking at the source code just makes me dizzy.