In winter Pari wore one thick woollen sweater and her brother wore two thin layers — a t-shirt and…

Why

Pari's assumption is a fair one, and often correct — a thick sweater usually does trap more warmth than a single thin one, so 'thicker means warmer' has real experience behind it. What decides warmth is really how much still air gets trapped close to the body, and layers are surprisingly good at exactly that. Each thin layer holds its own thin cushion of warm air, and stacking two layers stacks two cushions — sometimes trapping just as much air overall as one thick layer, or more. That's why her brother, in two thin pieces, felt just as warm as Pari in one thick one; the total trapped air ended up similar. Test: this is why mountaineers wear several thin layers rather than one giant coat — it traps more air AND lets you peel one off if it gets too warm. No, several thin layers can work just as well: warmth comes from trapped air, and layering is one good way to trap plenty of it, not only thickness.