The adorble-bordering-on-gross behavior of rats when they are happy. The beady eyes stick out as the lower jaw goes back and forth, grinding the teeth in contentment. The proper name for this behavior is bruxing.
Aww, Bom Bom must love getting groomed by Zoey: just watch him eye boggling those peepers back and forth, back and forth, in and out, in and out. You can hear the reeaaly loud teeth grindage!!!