Q: My puppy bites at my loose trouser leg and shakes it. I tried growling and shaking my leg, which made it worse, so I tried ignoring it, but that hasn't worked either. What can I do?
A: When a puppy bites a loose trouser leg and starts shaking, it is usually a self-reinforcing behavior.
It's fun for puppy—this is a game of tug—so he does it more often. Growling and shaking your leg will excite some pups even more, and ignoring the behavior doesn't mean that the behavior isn't being reinforced.
One possible solution is to teach your pup to play tug, but by the rules and with a more appropriate item than your trouser leg. This would give him plenty of opportunity to play the game he obviously enjoys. When your pup knows how to play tug by the rules, you can use tugging as a reinforcer for other behaviors.
Complete instructions for using tug games as a novel reinforcer can be found here.
Try to figure out when your pup is likely to grab at your trouser leg. Often, this is while walking on leash. If that's the case, teach your pup to walk nicely on leash using clicker training. After you click, use short games of tug as the treat.
Reinforce the behavior you want with behaviors that your pup wants.
If your pup grabs at your trouser leg after you have trained him to play tug by the rules, ask him to "'out." Reinforce the "out" with a game of tug.
Of course, some pants are just asking for trouble. If you have pants that flap around excessively, don't wear them around a young pup—save them for other occasions. Why waste time solving a problem that you needn't have created in the first place?