How to affect other elements when one element is hovered

Question

What I want to do is when a certain div is hovered, it'd affect the properties of another div.

For example, in this JSFiddle demo, when you hover over #cube it changes the background-color but what I want is that when I hover over #container, #cubeis affected.

div {
  outline: 1px solid red;
}

#container {
  width: 200px;
  height: 30px;
}

#cube {
  width: 30px;
  height: 100%;
  background-color: red;
}

#cube:hover {
  width: 30px;
  height: 100%;
  background-color: blue;
}


<div id="container">
  <div id="cube">
  </div>
</div>

Answer

If the cube is directly inside the container:

#container:hover > #cube { background-color: yellow; }

If cube is next to (after containers closing tag) the container:

#container:hover + #cube { background-color: yellow; }

If the cube is somewhere inside the container:

#container:hover #cube { background-color: yellow; }

If the cube is a sibling of the container:

#container:hover ~ #cube { background-color: yellow; }