To begin the process of sending flowers to my friend, I give Flo a message detailing the type of transaction I desire. Flo accepts my message, and gives me a receipt.
In turn, Flo gives another message to the florist in my friends 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.