What makes Live Rock "Live"?
 

My brother had just asked me while flipping through a catalog and I truthfully didn't know the answer...anyone?

i believe you want this in the "saltwater section" for better responses so i went ahead and moved it for you.

live rock is simply covered in life. it has all sorts of ciritters. ampipods, copepods, astrina starfish, bristle worms and so forth. depending on where the rock is aquired it can also include pests like mantis shrimp, aptasia anemones, flatworms and so forth. live rock is one of the main sources of filtration (along with protein skimming) in a saltwater tank. good flow blowing water against these oraganisms allows it to "filter" naturally.

gotcha, thanks.