A nice metal pen, a mobile phone, spare change, headphones, custom jewelry, a chunky watch, car keys, house keys, hiking boots....
You may have this on you when you are out and about.
You don't need to be a ninja to work out how to use them
You do need to practice using them though...
However you could just attach a razor blade to the bottom outside edge of you boots by the heel and spray it grey/black so it does't show up easily on pavements and tarmac. Works well with cutting achilles tendon or removing knee caps
