Show several razor blades to your audience, and show that they are apparently very sharp. The audience sees you place these blades, one at a time, in your mouth. It appears that you are actually ingesting them! Afterwards, you show your mouth as being completely empty!
This is a perfect opener before you perform the classic effect, Threaded Razor Blades From Mouth.
Can be performed in street magic, a restaurant, or even as a fakir demonstration.