Rats grind their teeth for two reasons:

First, a rat’s teeth never stop growing; they keep the length under control by constantly grinding their teeth and gnawing on hard objects (this is why your rat may keep you awake at night chewing on the bars of his cage).  Second, rats often grind their teeth when they’re happy!  It’s called “bruxing.”  You may also notice their eyes bugging out while they do this…it’s called “eye boggling,” and if you’d like to see a video of this phenomenon, click here!