As much as we love gadgets, gag props can be spotted from a mile away. And the last thing you need when you are fake self-beheading is some little snot pointing out that the knife is plastic. That's why this bleeding trick may be our favorite. Instead of using tin foil swords and bags filled with dyed corn starch mixtures, the stunt utilizes chemical reactions on your skin. Just make sure you are using the dull end of the knife, lest the effect be highly realistic.
Will people please stop calling tricks "hacks". You're not hacking your blood. I think the Mountain Dew pranksters used the word "hack" also. I guess I can start calling David Blaine a really good hacker. Probably the best hacker I've ever seen.