How can I make a div stick to the top of the screen once it's been scrolled to?

Question

I would like to create a div, that is situated beneath a block of content but that once the page has been scrolled enough to contact its top boundary, becomes fixed in place

Answer 1

The accepted answer works but doesn't move back to previous position if you scroll above it. It is always stuck to the top after being placed there.

  $(window).scroll(function(e) {
    $el = $('.fixedElement');
    if ($(this).scrollTop() > 42 && $el.css('position') != 'fixed') {
      $('.fixedElement').css( 'position': 'fixed', 'top': '0px');

    } else if ($(this).scrollTop() < 42 && $el.css('position') != 'relative') {
      $('.fixedElement').css( 'relative': 'fixed', 'top': '42px');
//this was just my previous position/formating
    }
  });

jleedev's response whould work, but I wasn't able to get it to work. His example page also didn't work (for me).