I have the Art of Astonishment by Paul Harris and in the trick Pain I need a straight pin. How large should the head of the pin be for the trick to be convincing without me impaling myself? Because if I use one with a large plastic head I doubt it's going to trick a five year old, and if I use one with a very small metal head I'm kinda terrified of needing to go to the ER to get the pin removed...