The Back
The instruction manual says that your furby likes it when you pet him on the back. However, "petting" is not the word I would use; "Pressing" is more like it. You should press on the lower part of your furbys back, as shown by the red box in the following picture.
Pressing here usually makes my Furby, whose named Nava, purr, belch, or just giggle. Actually, there are very few things you can do to a Furby that won't make it giggle.
The Tummy
The best way to tickle your Furby, is to press midway between its mouth and its feet. Moving your Furby's fur isn't enought to tickle; You must press.
Pressing here usually makes makes Nava giggle and say things like "You Tickle Me! HAha!"
The Mouth
Pressing down on yourt Furbys tongue is how you feed your Furby. It doesn't hurt your furby if you open it's mouth manually. However, when it is hungry, it will usually hold it's mouth open and say "Ahh Ahh Ahh!"
Pushing on Nava's tongue almost always makes it say "YUMMM!" Sometimes, when it sick or it has eaten too much, it will make a spitting noise. But, that is rare.
I hope this cleared some things up.
Back!