Dogs are known as friendly, tail-wagging creatures that will do almost anything to please their owner. However, every animal can act out when it's scared or placed in an unfamiliar environment. Foster Wong, a certified professional dog trainer from the American Karen Pryor Academy, provides insights on how to handle a canine that suddenly turns aggressive. 'Aggressive behaviour in dogs is very common. It's definitely one of the top five issues owners have, on top of other issues like pulling on the leash, toilet training and barking,' says Wong, who practices positive training methods. 'The reason for aggression - lunging, growling and biting - is that dogs are acting out of fear.' As aggression can be classified into different areas, such as dominant, possessive and maternal, Wong sees fear as the most common type. One example, he says, involves dogs that are not used to children running around and shouting. 'Some parents don't realise there's an issue with the dog and let the children say 'hello'. But if the dog is on a leash, it has no way to escape,' Wong says. 'In these kinds of situations they want to fight or [take] flight. Every dog has its comfort zone of personal space and if you burst their comfort bubble, the dog will react. Sometimes the only thing they can do is bite.' Another factor on whether a dog will become aggressive depends on a particular situation. Wong gives an analogy of personal space for people. If you are sitting on a bus that is crowded, and a person sits next to you, most people wouldn't react. However, if the bus were empty and a person takes the seat right next to you, it would feel awkward and strange. Wong says: 'Dogs don't always act out. It depends on the context. Your dog may be OK with John, but not OK with kids in the park.' If your dog is in the middle of eating a tasty treat or morsel, only the bravest, or most foolish, person would try and take it away. In this situation, possessive aggression is shown through dogs naturally protecting their resources, whether it's a favourite treat, toys, food or a person. 'The dog may bark and growl to fend you off from a valuable resource. But if you ignore this warning sign he will snap at you,' Wong says. 'It has nothing to do with whether you are a strong pack leader or not, it's just a natural behaviour to protect its resources.' According to Wong, possessive aggression doesn't always involve a physical item, it could also be a particular spot that warrants the same reaction of barking and growling. As every dog's personality is different, the trainer believes canine behaviour is 50 per cent innate and 50 per cent learned. For dogs that have been raised on Lamma or Lantau Island, for example, they may have never been exposed to busy traffic. If then, you place a country canine in the middle of Central, with bumper-to-bumper beeping cars and crowds of people, the dog may become fearful. Another dog may have never seen a Schnauzer before, thinking it's a monster. Wong says: 'If in a fearful state, the dog makes a negative association and something nasty happens, all this adds up to is fear aggression.' To learn positive associations, it's important to socialise a puppy between four and eight months of age. 'This is the golden learning opportunity when a dog's brain develops. You should introduce novel objects, sounds, people and other animals,' Wong says. 'But after eight months, the window of learning shuts. You can teach your dog to not fear certain things, but it will take longer.' The best way to tackle your dog's aggression and fear is to understand the underlying reasons behind it. Wong suggests answering the five W-questions: who, what, where, why and when? What situation triggered the aggression, and then try to change your dog from the outside in. 'If your dog is scared of strange men, then try to make a positive association by counter conditioning or turning the things around,' Wong says. An owner could set up a scenario with a friend who gives your dog a treat or anything that the dog enjoys to gradually make a positive association with strangers. For dogs that have a history of biting people, Wong recommends avoiding crowded areas until the dog has been fully trained to be safe around people. Don't punish your dog for its aggressive behaviour as 'this will make things worse'. If a dog knows it will be punished for acting fearful, the usual warning signs of barking, growling or lunging may be suppressed. Then, fearful dogs may skip the warning stage altogether and go directly to biting or snapping at people.