Hi guys

At the moment i'm playing Half life 2 and i must say that it's the best game i've played so far. Especially the graphics and the physics are great. When you try to destroy/damage something with e.g my crowbar, a hole appears in the object.
the HL2 engine uses parallax-mapping to make these holes look realistic (with real depth, while it's just a flat texture).
So that raises a question:
How can i add damage to an object. (doesn't need to be with parallax-mapping). Just a hole that appears on the 'damaged place'.
How do they do that?? Is it some kind of multitexturing??

Can someone explain this?? :?