To begin the process of sending flowers to my friend, Chris gives Fred a message detailing the type of transaction desired. Fred accepts the message, and gives Chris a receipt.
In turn, Fred gives another message to the florist in Robins city. This florist gives yet another message to the flower arranger, and in time another message to the delivery van driver.
In each case the service is initiated by making a request on an object for it to perform one of the services that it provides.