Try the punch tool out on a glass bottle or jar. Doing this will give you an idea of how it works. Obviously, you want to do this inside a trashcan or paper bag to contain the glass shards. Be sure to wear safety glasses , as jars and bottles are not safety glass , and will shatter differently. I used this method to test my S&W 911 Rescue Tool. Unfortunately, I have the earlier version with the plastic liner lock and the more dull, less curved blade. It'll still punch through any glass, though. <img src="images/graemlins/smile.gif" alt="" />