.collider{
  background: #f0f0f0;
  border: grey 1px solid;
  width: 500px ;
  height: 500px ;
  display: inline-block;
}
.ball{
  border-radius: 50%;
  position: absolute;
  transform-origin: center;
}
.ball__fill{
  border-radius: 50%;
  position: absolute;
  transform-origin: center;
}

.header{
  font-size: 32px;
}