EFFECT: You display a small bag with a spare button and some cotton thread inside, just like the kind you would get with a new shirt or jacket. You hold the bag to the front of your shirt/coat/jacket/hoodie (whatever you're wearing) and give a little rub.
Incredibly, the button and thread penetrate through the bag and magically sew themselves right on to the fabric! But, you don't really want a button sewn in such an odd place... it looks weird! So, you very visually (and quite casually) 'pop' the button over to a more discreet location on the inside of the garment! They actually SEE it move to a different place and everything is completely examinable!