Message in a bottle. Or, in this case, a box. This glowing box contains a Time Lords thoughts and a message which travels through time and space to get to a fellow traveller. It even knocks on the door. How very polite. Pulling a set of white cards out of his pocket, the Second Doctor constructed a hypercube and put a psychic message into it, sending it off to Gallifrey to ask the Time Lords to help him out in The War Games. And help they did, before they arrested him and dragged him back home to answer for his behaviour, that is. Returning in The Doctors Wife with the Eleventh Doctor, these seem to be Time Lord emergency distress beacons of sorts. Eleven found a cupboard full of them and was disheartened to learn that the owners were long since dead. Still, its a neat thing to have about your person, but fans dont suppose the Doctor keeps one handy any more.