Also the answer can't be get fatter because especially in teens the hands tend to not want to grow.
What are the best exercises?
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I suggests grip training (crushing, gripping, pinching ...), forearm training, climbing and hard physical labor. Some scientific researches have found that competitive climbers hands have thicker and denser bones, muscles and tendons. That is due to the body adaptation to externals stimuli. In the same way woodworkers, lumberjacks, smiths and many hard workers develop strong and bigger hands