span.tool {
  position: relative;   /* this is key */
  cursor: help;
}
span.tool span.tip {
  display: none;        /* so is this */
}

/* tooltip will display on :hover event */
span.tool:hover span.tip {
  display: block;
  z-index: 100;
  position: absolute;
  top: 1.6em;
  left: 0;
  width: auto;
  padding: 3px 7px 4px 6px;
  border: 1px solid #336;
  background-color: #f7f7ee;
  font: normal 0.9em/1.2em arial, helvetica, sans-serif;
  text-align: left;
  color: #000;
}
#content span.tool::after {
  padding-left: 2px;
  content: url(/img/bubble.gif);
}