ANDY'S ANSWER: Your hormones start going into overdrive during puberty and one of the results is that your body will heat up and in turn, you will sweat more. Sweating is your body's natural way to help cool you. (The moisture evaporates from your skin producing a cooling effect.) You actually sweat from up to 5 million sweat glands which are distributed over your whole body. Sweat itself is mostly water and doesn't smell bad. The reason that your feet stink and your underarms can get rank is because evaporation doesn't happen as efficiently there. The sweat combines with the bacteria on your skin and ferments to give that lovely aroma that makes people's eyes roll to the back of their heads.
Your best defense is to wash or shower often, especially after you have been sweating a lot. Wearing loose-fitting shirts and using deodorant will help, and if your sweating is extreme you might want to wear antiperspirant instead of deodorant.
For your feet, again wash well with soap and water, particularly between your toes. Change your socks often if you are wearing shoes. Wearing sandals or going barefoot when you can will also cut down on the odour. Letting your shoes dry between wearings (like not wearing the same shoes every day) or washing them if you can is also helpful. There are also foot powders that you can sprinkle in your socks to keep your feet drier.
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