Things dogs like to chase

When we see Jack staring out the window with his ears pricked up it might just mean there's a goofy looking squirrel running around in Jack's territory, and if the dog is in the house when he sees it then it's all right. He will probably only bark, growl, and whine miserably. If we're taking Jack on a walk when he sees the squirrel, however, Jack will very likely pull desperately on his leash, and run with all his might at the silly rodent with the long, fluffy tail.  
 It can be really annoying when Jack does this, and that's one good reason to teach your dog to 'heel.'
Jack isn't good at 'heel' though, so it's easier to walk him with a short lead, that way he can't gain momentum by running full speed in the direction he wants to go and then yanking really hard on the leash.
Dogs like chasing small animals; especially squirrels, rabbits, cats, and chickens who flap their wings too much. Jack likes chasing the chickens when they flap their wings for some reason.
Some dogs like to chase cars; if they do this, they are liable to get their paws run over.  Eventually our dog has pretty much learned to stay clear of things with wheels.

                                                           "Hmm, are there any
                                                             squirrels out here?"