 Is anyone from outer space trying to make contact with us? A science fiction writer called Carl Sagan suggested that if someone was, they might send us a picture coded as numbers. In fact, we already have digital systems for sending pictures as numbers. One of them is called a fax machine. Suppose you wanted to send this picture over a long distance. Here it has been broken up into lots of black and white squares, called pixels. Rather than transmit every pixel separately, we can speak things up by grouping pixels in runs of the same colour. Here we have a run of three white pixels, then four black ones, then one white. Now the picture is represented with a whole lot of numbers, which makes it easy to store and transmit. Here we're going to send this picture from the paper to the side of a wall. The printer at the other end is a student who will paint black and white pixels for us. The first message is the number of white pixels to skip. Then we send a message to colour in three black pixels. We carry on with alternating numbers for white, then black pixels. The person sending the image just needs to count the number of white squares until the next black one. Then the number of black squares until the next white one. It's always alternating white and black. These pixels are more than a thousand times larger than the ones on a fax machine. Even though we've only sent a few numbers, we've communicated the whole picture. Of course, in class you don't have to use a sheet of paper this big. Fax machines use a similar principle for sending pictures. If they send each pixel individually, they'd be about seven times slower. And if you need to contact someone in outer space, perhaps you'd want to use the system too. For more information about this activity and the Unplugged project, visit our website at csunplugged.org.