How do I make a door that opens when two parts touch?

I am trying to make a puzzle game where you have to push a cube onto a button and once you get the cube on the button I want the door to open and I want it so if you take the cube off the button the door closes. (Like in Portal) If anyone could help me with this that would be greatly appreciated. The name of the part is name is Cube and the button name is Button.