“Ninja rocks” are little bits of spark plug porcelien. When throw against a glass surface, they shatter the entire window with only a small noise. It’s much less noisy than throwing a rock through, then having to scrape the insides of the window.
Possessing a ninja rock is “illegal”, as it is a burglary tool under the American system. Also remember the rock may have partial prints on it, as well as DNA. Folks who don’t collect it before leaving the scene….…. then disposing of it appropriately after removing all traces leave fingerprints that help HEROCops!!!™ find them.