NORMAN — When a dunker breaks a backboard, the glass shatters into thousands of tiny pieces, few larger than a thumbnail and, ideally, none in sharp shards. That’s because the glass used in backboards is tempered glass, different from the glass used in kitchen glasses or windows in the home. The half-inch-thick sheet of glass used for backboards is made by processes that place the glass under constant stress, strengthening the glass considerably. Tempered glass has strange properties, as evidenced by a "Prince Rupert’s Drop.” To make one, molten glass is dropped into cold water to harden, and develops a long, thin tail. "You can hit the large drop with a hammer and not shatter it, but damaging the thin tail shatters the entire glass drop,” said OU physics professor Stu Ryan. The bulbous tip can withstand heavy pressure, but snapping the tail produces a violent, shattering reaction. In normal-shaped pieces of tempered glass, however, failure is difficult. "Failure typically occurs at the edge of the glass or at a hole cut in the glass,” Ryan said.