Use this to target the "selected" element, then select the child with find() or children():
$(document).ready(function() {
  $('.box').mouseover(function() {
    $(this).children('.hide').show();
    $(this).children('.show').hide();
  });
  $('.box').mouseleave(function() {
    $(this).children('.hide').hide();
    $(this).children('.show').show();
  });
});