When temperatures drop below 18C, snowflake patterns will appear on this thermochromatic Jack Frost scarf from Diana Eng. But that's like using a bandage that tells you you're bleeding, or a hat that points out your hair's a mess.
Unless you leave this scarf hanging outside your window all night, it can't really tell you that it's cold enough to actually need a scarf until you go outside wearing it. And at that point you're already dressed for the weather, or uncomfortably overdressed.
Of course the scarf is supposed to be considered more of a whimsical fashion accessory than a cotton thermometer. But a scarf that actually had the ability to tell you if it was needed, or an umbrella that predicted rain, would certainly be embraced by geeky fashionistas. [Diana Eng via CNET]