If you drop molten glass into a bucket of water, it will solidify into what's called a "Prince Rupert's Drop." According to this Corning video, the surface of the drop is in a state of great compression, while the interior is in a state of great tension. You can squeeze the bulbous part of the drop with pliers or bang on it with a hammer to no avail. However, if you snap off the hair-thin tail at the end of the drop, it'll shatter into dust. (Via forgetomori)