For most of my life, fat was a number. A percentage. A thing to reduce, redistribute, be ashamed of in summer. I had no idea it was an organ — several organs, actually, each with its own cellular structure, its own hormonal language, its own metabolic personality. Brown fat burns. White fat stores. And beige fat, the one nobody mentions, can switch between both depending on the signals it receives. What nobody told me is that women have a thermogenic advantage built into their adipose biology. Until estrogen declines. Then everything changes at once.