The Pet was my first experience with networking. All the Pets in the room were connected to the same printer with Centronics cables (each plugged into the back of another). The printer was written to by outputting a byte to a port. I decided to see if it was possible to read the port and see the bytes that were being written by one of the other computers and it was.
Of course, "real" networking was already around by then but I thought it was cool.