Glass

From ibikeu Wiki

In the context of cycling glass is usually in reference to broken glass on the road. Glass is one of the most common road hazards to result in a flat tire for a cyclist. Glass can often be hard to spot when finely ground, but can be given away by reflected light. Because of the difficulty in spotting glass it is very important in group riding situations to call out glass to others who may not be aware it is there, with a pointing of the hand to indicate the location.

Be aware that glass bottles can sometimes be found laying in roads or bike paths hidden inside a plastic or paper bag. Unless you can clearly see that it is empty, assume that the bag has a glass bottle. Do not roll over it, or you may just be surprised with the terrible sound of your tire's death.