Using very cheap and little components (high voltage transformer + 18650 battery holder + 2 plug pin) you can make a very powerful stun gun (taser) for self-defense. All the material combined is less than $10 USD to buy them all. The high voltage transformer claimed to generate around 400,000 volts from 3.7 battery - although this might not be exactly true but the transformer is quite powerful indeed. The spark / arc generate very loud clapping sound which can shocked someone painfully.
DIY Taser | How to make stun gun | Cheap Taser | Powerful Homemade taser | High voltage homemade stun gun | 3.7 volt to 400,000 volt stun gun