The Daily Mail writes that one tourist, Mr. Wang, said that “the monkey was sharpening the stone, then it started hitting it on the glass. The monkey scared itself away, but it came back to take another look and even touched it.” Was what the monkey did simply an accident? Or was the Capuchin only pretending to be scared at first, so we wouldn’t suspect it had other nefarious plans?
This is also why tempered glass is used in car windows (as "safety glass"; it doesn't make long sharp "swords" that can impale you in an accident.