Hold your thumb at the base, right where you can start to bend the can in. Now, lower the can towards the ground so that it's somewhat upside down. Finally, and this part is really important, pull up and back really fast and headbutt the back of your thumb.
Boom. Thumb gun.
bitches love it when they can see my dick bulging against my outerwear. they are all like "is that a sandwhich for later?" and im all like "no, that is my massive penis sandwiched between the tight fabric of my pants and my inner thigh" - pomme-de-terre