Http requests

Could someone tell me how instant messaging works using http requests? If there are two clients, and a server can only respond to http requests, then how does the server send the instant message to the passive client without a request from that client?

I am working on something for school and the instructor does not know. I really appreciate the help.

You might find this link helpful ( http://www.jabber.org/jeps/jep-0124.html ). It describes how XMPP tackles your problem. A couple of other solutions are briefly mentioned in its introduction.