One of the great breakthroughs in networking appeared in the late 1960s when the ARPANET was being planned. That is to break a message up into small chunks of bits, called packets, and send them over the network rather than try to send the entire data stream, as is done with normal telephones. Packets are now so ingrained in the thinking of computer scientists and network engineers that it is hard to imagine a time when the concept was invented. It is also hard to visualize what a packet really is. This section attempts to explain this.