How often do you use the flashlight on your smartphone? In a pinch it can be pretty helpful, and Endo75 has just figured out how to make it even more so. Using just tape and colored Sharpies, he creates a detecting tool that’s super useful and pretty disgusting. Fair warning— this hack is useful, but it might show you things you’d rather not know. Say hello to your very own DIY blacklight.

With Halloween right around the corner, this trick could work in a cooler (and more enjoyable) way for you and your kids. Create the blacklight effect using your phone, and use it like DaveHax does to bring your Halloween party to the next level: