See Thru Magic Guillotine Finger Chopper
You show a clear plastic finger chopper with a hole for the finger and a hole at the bottom for a carrot or something similar. You show that anything placed in either of the hole will be cut by the blade inside the finger chopper, yet when you place your finger inside the finger chopper, miraculously the blade passes right through the finger without hurting your finger. This model of the Finger Chopper has a unique locking mechanism and is examinable! We have shown it to dozens of knowledgeable magicians, and they have not figured the clever secret. Supplied ready to work, with instructions.