I'm about to let you in on my best hack for staying cool indoors when it is unbearably hot in your home. This hack isn't altogether pleasant, so I mostly advise it in situations when there's a heat advisory. My first line of defense is a cold shower, however that's not completely practical (there's only so many you can take a day) and the effects don't last longer than 10 minutes in 100+ heat.
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So, my trick is this: Submerge a T-shirt in cool water, wring out, and then wear that T-shirt inside your home. It might not feel great, but it will keep your torso cool for a good half hour or so before the shirt completely dries out. Of course, during the time you're wearing the wet T-shirt, you might not want to recline on any upholstery, so it's something I like to do when I'm cooking or fussing about, or chilling out in my dining room, which has non-upholstered chairs.
Trust me, it definitely offers relief from the heat.