The grates that protect prison windows are often made of pipes with a bunch of loose rods inside. Cutting through the pipe itself is relatively easy, but the ro

The grates that protect prison windows are often made of pipes with a bunch of loose rods inside. Cutting through the pipe itself is relatively easy, but the rods inside are unfastened, so they tend to roll under the cutting tools. Edit: The principle is actually somewhat similar to the anti-drill protection on door locks - a small disc in front of the lock with a slit that the key goes through. The disc makes it hard to drill through the lock, because it spins together with the drill.