Tattoos are pretty permanent decisions, and laser removal is often described as more painful than actually getting them in the first place. But what about a design that can only be seen when you want to see it? Engineers at the University of Colorado Boulder are working on a new type of tattoo ink to answer that question. Their dye is specifically concocted to physically alter depending on exposure to visible or ultraviolet (UV) light.

The team described their invention in a study published earlier this month in the journal Matter & Light, and they didn’t only demonstrate the capabilities in the lab on silicone and pig skin. One of its creators, Carson Bruns, has already tattooed himself with the ink.

“[It’s] almost a tradition,” the study co-author recently told Nature.

Bruns, a nanoscientist at UC Boulder, previously developed a now-commercialized tattoo product called “Magic Ink” that turns magenta depending on its exposure to UV light. To highlight its efficacy, Bruns gave himself “solar freckles” as a method to track his exposure to sunlight. In similar fashion, he recently completed a similar session on his leg using the disappearing ink.

The formula currently comes in three colors—yellow, blue, and magenta—and essentially relies on the same principles behind transition lenses and some textiles. Each dye is composed of tiny spheres of acrylic glass. These beads contain photochromes, which are chemical compounds engineered to absorb specific light wavelength photons. In this case, the photochomic dye’s spheres begin in their original color, but then flatten into transparency after absorbing corresponding light photons exposed to UV rays. The dye remains this way until it absorbs visible light photons, which destroys the chemical bond and allows the spheres to return to their original shape.

Aside from the lab tests, Bruns tattooed his own leg over about 12 months using various dye combinations, all of which responded as intended to UV and visible light exposure. The ink also remained effective for over two years—but the team’s intention is go beyond a nifty parlor trick. Future ink iterations may be particularly useful in medical procedures, such as temporary markings to help maintain accuracy during direct radiotherapy sessions. For now, however, the photochromic tattoo dyes remain for laboratory (and Bruns’) use only until further safety assessments are completed.

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Not far from the Black Sea, a child’s grave uncovered in present-day Bulgaria has provided archaeologists a somber look at Roman funerary practices. Flagged in the city of Stara Zagora during a site survey for a new apartment complex and parking garage project, the burial contained stunningly preserved gold earrings as well as a single, small coin known as an antoninianus. But while the currency’s age is traceable to within a year of its minting, the bronze piece may also represent a grieving family’s hopes for their child’s safe passage into the afterlife.

The Roman empire at its height included over 1.9 million square miles of territory, including much of central Europe and central Asia. Rome annexed the region of Thrace in 46 CE, and eventually renamed the city of Stara Zagora as Ulpia Augusta Traiana in honor of Emperor Trajan (98 to 117 CE). Augusta Traiana quickly grew into one of Thrace’s most prominent urban hubs, becoming so prosperous that Rome ultimately granted permission to build its own coin mint during the second and third centuries CE.

Over 1,750 years later, Augusta Traiana’s remnants exist underneath a city once again known as Stara Zagora. Ruins of the Roman empire are regularly unearthed through the town, including the recent discovery of a large cemetery used for nearly three centuries outside Augusta Traiana’s original walls. Among the 135 graves, researchers identified a child buried with a pair of gold earrings and one of Thrace’s bronze coins. Although details about the young individual’s death and identity remain unclear, the coin clearly dates to either 274 or 275 CE. Archaeologists assume the child likely died either around that time period, or soon after.

While researchers may never learn the precise circumstances of the burial, ancient Greek and Roman funerary beliefs suggest the coin served a specific purpose. It’s possible the child’s family intentionally placed the currency with the deceased as payment to Charon, the ferryman who brought souls to the afterlife. Knowing this, it may be that grieving relatives hoped to impart one final gift to their loved one.

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Snakes will bite, rattle their tails, and even spit venom to escape danger. But of all the world’s species, southeast Asia’s dwarf reed snake (Pseudorabdion longiceps) relies on an especially fascinating escape method. When threatened, the diminutive serpent rolls away like a speeding wheel.

An adult dwarf reed snake typically measures less than 12 inches long, but its unique locomotive maneuvers allow it to retreat as fast one foot per second—about four times faster than it travels by simply slithering on the ground. It’s not a flailing getaway, either. The snake also easily rolls along a straight path while somehow keeping its head both stable and facing forward.

Now, researchers finally have a better understanding of how the dwarf reed snake achieves such dizzying acrobatics. In a study published today in the journal Current Biology, a team at Japan’s Toho University detailed the specific physiology that enables its locomotion after filming the snakes with high-speed cameras. Essentially, the snake doesn’t rotate its entire body in a singular, forward motion—it whips one side of its body forward towards the ground while an arched portion swings down by its head in what the study’s authors call a “stroke.” The snake then alternates between its left and right side coming into contact with the ground for each successive stroke. Essentially, it’s like twirling a rope in a lateral circle while holding one end in your hand—except that end is the snake’s head.

The cumulative efforts bypass the traditional idea of a wheel requiring something like a central axle. Instead, this ingenious, highly controlled movement allows the animal to keep its head and sensory organs upright and facing ahead while traveling at such high speed. This is compounded by the dwarf reed snake directing regions of its body to engage in large, alternating rotations while stabilizing its head.

By gaining better insight into these biological mechanisms, researchers also highlight the difficulty in trying to predict an animal’s capabilities based solely on its physiology. A snake may have primarily evolved to slither—but that doesn’t mean it can’t get creative when necessary.

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