physics - Basic Box2D collision detection

I don't understand how to listen for collisions in Cocos2D/Box2D. Say I have two dynamic circle bodies. One is very small and the other is relatively large. When the small circle collides w/ the large circle I'd like to do something (play a sound for example). What's the best way to do this?

I'm currently experimenting w/ the TestPoint method. Something like:

if(largeCircleBody->GetFixtureList()->TestPoint(smallCirclePoint)){
    // collision happened...
    // play sound etc
}

Answer

You use b2ContactListener

http://www.box2d.org/manual.html#_Toc258082975

http://www.emanueleferonato.com/2008/12/23/understanding-how-box2d-manages-collisions/