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One of puberty's clearest indicators is when you start getting hairs in unexpected places. For boys, it is likely to include facial hair, underarm hair, more hair on the legs and chest, and pubic hair. Girls are likely to spot hair growth on the underarms and in the pubic area. Acne is a common adolescence symptom which is caused by increased secretion of oils and grease in the pores, and bacteria infecting them. If you've got acne, you're almost certainly going through puberty. You may find that it took you a year to manage a couple of inches before, you're gaining inches in height faster than you can count them! If this is you, and you're 10-13, you've probably started puberty.  If your appetite suddenly takes a leap like it's never leapt before, that's a sign of puberty, too. It may sound gross, and it's true - the smell is hardly appealing. If you find you get stronger-smelling sweat and more of it, even when you're not actually hot, you've probably started puberty.
Look out for hair growth. Look for acne. Think about your growth and energy. Look out for signs of more sweating.