//cube is directly inside the container:
#container:hover > #cube { background-color: yellow; }
//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; }
div {
display: none;
}
a:hover + div {
display: block;
}