Brilliant! A handmade rearview mirror for bicycling. For whenever you need to see what's happening behind you, or just need a constant reminder of what brand of sunglasses you're wearing.
While Bikes and the City doesn't give complete step-by-step instructions, a quick look at the photos intimates the process:
First, bend an eight- or ten-guage piece of wire so that it hangs securely on you glasses or from your helmet. Use an awl or small drill bit to punch a hole through the bottle cap, and secure with hot glue. Then, glue in a small round mirror from the craft store, and hit the road!