I have a pet iguana. when holding it ,sometimes it starts wagging its tail side to side . anyone know why it d

This is a very obvious form of body language. Whipping its tail is usually the first weapon an iguana will use to protect itself. It's usually just a painful experience, but preventable none-the-less. They usually have very good aim and the speed of a good tail whip will amaze most people who have never experienced it. You can't stop an iguana from whipping its tail at you, but you can prevent being hit by it. First of all, they will usually show other signs that they are irritated before cracking the whip. Secondly, there may be a twitching or wagging of the tail before it attacks with it. If you see the signs, you should be safe. If not, the damage it can cause is usually not too bad, except for a welt and maybe some hard feelings, although a good aim at the eyes could be very bad...so be careful!