If you think you know, or you really know, then you are an awesome parent. But, is all that you know actually the truth. I can't say about other countries, but back here in India, teenage kids actually live a dual life. Most of them have a hidden life which their parents are unaware of. I have seen kids who smoke and waste themselves to useless substances and I am pretty sure that their parents know nothing about it because if they did then they surely would try to get them out of it.
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Why do kids hide secrets from parents? Why don't they confide in parents? Well, that's here where we fail as parents. Parents should be more like friends to their children. We parents get so busy in making them perfect human beings that we forget their raging harmones will make them rebellious. Instead of restricting them from doing certain things, you should explain to them the pros and cons of doing so. But, then to teach them you need to know what they are up to and the only way to know this is to be friends with them. Go back to your teenage years when you are with them. Tell them about your teenage experiences and it's effects on you.
Life is all about doing mistakes and learning from it. Nobody is born perfect. Don't control them, just train them. But then, saying is easier than doing. I hope I will be able to practice what I preach. I still have 3 years for my elder one to enter teenage. I am learning and practicing.