A kotatsu (炬燵) is a low table with a heater built into its frame, covered by a thick blanket and topped with a flat tabletop. Slide your legs underneath, and the blanket traps the heat around them while the rest of the room stays exactly as cold as a Japanese winter usually is. For a few months every year, the kotatsu becomes the one place in the house nobody wants to leave.
Heat where you actually sit
Japanese homes have traditionally been built to breathe with the seasons, kept open and airy for the hot, humid summer, which leaves them cold and drafty once winter sets in, since central heating never became the default the way it did in colder countries. The kotatsu solves that with a simple, local trick instead of trying to warm a whole room: heat just the small space where a person actually sits, trapped under a blanket thick enough to hold it in. Older kotatsu sat over a small pit dug into the floor with hot coals inside; modern ones use an electric heater fixed to the underside of the tabletop, thermostat and all, but the basic idea hasn't changed at all.
The gravity of a warm table
Once winter arrives, a kotatsu tends to become the household's main gathering spot: people eat there, watch television there, do homework there, and more than a few simply fall asleep there, legs still tucked under the blanket. Getting up to fetch something from across the room takes a small act of will once you're settled in, a feeling well known enough in Japan to have its own bit of dry humor about it. Cats love a kotatsu every bit as much as people do, and a family pet curled up half inside the blanket is one of the most familiar sights of a Japanese winter.
Words & idioms to take away
Idioms & proverbs to carry away
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布団 (futon): here, the thick blanket that drapes over a kotatsu's frame and traps the heat, the same word used for Japanese bedding elsewhere in the house.
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蜜柑 (mikan): a small, easy-to-peel mandarin orange, the classic snack eaten while sitting at a kotatsu in winter.