So, if you've learned nothing else from this, at least you know not to use your freshly hacked phone in the bathroom. That's something you just can't un-see. Still, this nifty DIY ultraviolet light has lots of practical (and fun!) uses. Counterfeit bills can be easily detected with a black light. Plastic security threads will appear under UV light in legitimate bills. Additionally, the threads will glow different colors depending on denomination. For instance, a $5 bill will be blue, a $20 bill shows up green and a $100 note glows pink. Your DIY black light can also be helpful if you have any pets. Sometimes accidents happen, and it can be difficult to identify the exact spot. A black light, however, will quickly illuminate the problem.