No tricks here, apparently cats have the anatomy to be able to bark, just as readily as meow!
So if you think you hear a dog around when only you and your cat are home, watch him or her carefully. You too may catch your cat barking!
Now hit this link and choose number 9 to discover and hear the cat bark: http://www.livescience.com/33400-8-weird-animal-facts.html
Interestingly, this is Whippet Boy's all time favourite animal clip. Try it out with your dog within ear shot and let me know if it looks shocked too!