Calvin thinks his name is CalvinNo. He can be the naughtiest. However I do believe they understand words. I've tried saying the same words in different ways and generally get the same reaction. So far he understands:
treats (no explaination needed)
flippa flip (where he does a semi tumble and flips left and right on his back..pretty hilarious
lie down (he doesn't ALWAYS do this but usually relents)
What's that? ( as in there's a bird/squirrel on the patio. I can say it from the bedroom and he'll fly to the sliding glass doors to take a look)
Where's Hobbes? (he'll go to wherever his brother is. If he doesn't know he'll look for him)
Mousie (he'll go get a mouse from his toybox if he wants too)

I think some understand more than others and some don't let on they know perfectly well what you are talking about! Isn't there that saying "If cats could talk, they wouldn't"!
Claudia