Bike Balls is a $13 rear light for your bicycle that dangles and swings from the back of your seat so it looks like your balls are glowing at night. The concept is that the movement of the light, combined with the fact that it looks like a glowing nutsack, will grab the attention of drivers who will then proceed to not run over you. Plus it makes a great opportunity for drivers with young children to explain what testicles are and what they do (what do they do anyways? I mean besides itch and get stuck to my legs in the summer?). Alternatively, just lie and tell them it's an upside-down heart and and then turn the radio up full blast so they can't ask any more questions.
Keep going for their Kickstarter video (project is already funded with almost six times their asking goal though because apparently there's a very serious demand for testicle shaped bike lights).