how do I make my thighs smaller?
Sometimes 'thicker' thighs are genetic, so take a look at your mum (or a picture of her at your age) to get an idea. Or, like me, your thighs could just use a little thinning up.
Being female, a high percentage of fat goes straight to our thighs and backside (but you already knew that). Hence, we first need to burn off that fat. Best way to do that, is through cardio exercise, and a proper, sensible diet.
You can either do high-intensity cardio in short bursts (e.g. sprint sets for 30 minutes a day) or low-intensity cardio for longer periods (e.g. running on a threadmill for an hour). Since I don't know what your health/physique/activity level is like, you need to consult your physician about which one of these options are better for you. He may also be able to give you a few more tips on burning off fat, including a good diet.
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Check out this link for exercise information:
running a lot... but not up hill, on a flat spot