The Internet (and most other networks) work by splitting data into packets. Each packet contains just a small part of the data along with the computer addresses of the sender and the destination. The packets are sent across the network, they can each take different routes to the destination computer where they are assembled into their correct order so their contents can be read.
We saw earlier how most nodes in the Internet are linked to many other nodes. When a packet reaches a node a piece of equipment called a router examines the packet, chooses a suitable route and sends it on to the next node. A packet can pass through up to 255 nodes (a number keeps count of each step), after which it is lost. The router has to decide what is the best route for a packet to take in order to reach its destination, which may be many steps away.
We can simulate a simple routing process with balls or fruit. It's a bit like 'pass the parcel'. Sit in a circle with a unique label attached to your shirt or at your feet. You will have two balls or oranges labelled with your identifier - one person has only one so the balls can be passed around. At the start you can hold any ball/fruit but your own. As the game progresses you can pass either of the balls/fruit you hold to the one empty hand in the circle. The aim is to acquire your own two balls/fruit and the game finishes when everyone has their own.
Deadlock is the situation where data cannot move through a network because two or more processes want the same item. Deadlock occurs in the simulation when someone tries to hold on to their own balls/fruit before everyone else has theirs. The simulation can only be completed if you cooperate.
An alternative exercise is to simulate computers and routers. Three quarters of a class act as computers sending and receiving messages in packets while the other quarter act as routers that send the message packets to their destinations. The computer pupils write a message across separate sheets of paper with their own address and that of the recipient in the 'header' of the packets. They take each 'packet' in turn to a 'router', the router examines the header and sends it on to either another router nearby or the destination computer.