Glow-in-the-dark spinning fiber
Who doesn't love stuff that glows in the dark? One of the most precious items in my stash is a cake of glow-in-the-dark yarn called Nightlights by Fiber Fantasy. Sadly this yarn has been discontinued. Nightlights is (was) a true glow-in-the-dark yarn: hold it up to a strong source of light, turn out the lights, and it glows in the dark.
By comparison, when a lot of people say "glow-in-the-dark" what they really mean is either "fluorescent" or "highly reflective." Fluorescent things shine under black light, the way that every single bit of lint on your black shirt glows when you go to a club with a black light on. And in the "highly reflective" category we have those reflective safety strips that you can stick to the back of your bike helmet or cycling jacket for safety after dark.
Nightlights has been discontinued, but if you're a spinner, you can buy glow-in-the-dark spinning fiber from Etsy seller Zauber Zeug. Fiber artist Kate Sweeney specializes in light-reactive fibers, and in addition to all the amazing fluorescent fibers she has for sale, she also sells glow-in-the-dark spinning fiber. This fiber is true glow-in-the-dark, but it's a hot seller - you may need to contact her directly to request a custom order. (Totally worth it!)
Don't hold your breath for spinnable glow-in-the-dark fiber directly off the sheep, though. Although there are currently nine glow-in-the-dark sheep living in Uruguay (the product of gene splicing with a glow-in-the-dark jellyfish), the glow effect is limited to the animal's body, and does not show up in its fleece.
If you want to add safety to your knitting - like adding stripes to a hat you wear when running in winter, for example - Metlon Corporation's RetroGlo is aimed as a knit-along product to add shiny to your knitting. The nylon tape is highly reflective on both sides, thanks to a 3M technology that "places thousands of glass beads per square inch."
Image courtesy Fundacion IRAU/J. Calvelo