Actually, a sharp pointed hard object will do it with much less effort. They sell these items at Sears and elsewhere that have a spring loaded sharp-ended rod. They say it works easily.
http://www.google.com/search?q=car+window+break+tool
I have one of these in each of my cars. It has a sharp point on one end and a razor (to cut open the seat belt in case of emergency) on the other. Better to be safe than sorry!