.bubble
{
  position: absolute;
  width: 50px;
  height: 50px;
}
.cur_bubble
{
  left: 360px;
  top: 470px;
}

.bubble_0
{
  background-position: 0 0;
}
.bubble_1
{
  background-position: 0 -50px;
}
.bubble_2
{
  background-position: 0 -100px;
}
.bubble_3
{
  background-position: 0 -150px;
}