It’s basically a type of oil/fat/lipid. It just kinda stays on your skin and prevents water from evaporating. So use it right after a shower to trap all the moisture.
Side effect is that (1) it leaves skin feeling oily because vaseline doesn’t really get absorbed and (2) oils are notorious when it comes to clogging pores. Your body’s own natural oils clog up your pores and cause acne, a synthetic oily lotion isn’t going to help with that for some people.
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