Generally, after 5-10 minutes, you and the absolutely astonished crowd gathered around will see the napkin start to "bustle" or vibrate at least a little. Give it another minute or two, have someone in the crowd do a drum roll, unwrap the napkin and watch in amazement as the "dead" fly flies away!
How can this be? Are you a magician bringing inanimate dead tissue back to life? Are you a modern-day Dr. Frankenstein performing morbid experiments on bugs?
Nah! You just possess a little knowledge of bugs and diffusion. A fly breathes thru "tubes" in the sides its body. Also, a fly, like most bugs, can exist for very long periods of time with little oxygen. It's a volume vs. surface area thing. So, the fly goes into a homeostatic state where it's sorta dead, but not really dead. It is just conserving what little oxygen it had before "drowning". The salt removes the water from its breathing tubes by a process of diffusion, more accurately "leaching", similar to cell diffusion, but without the membrane. With the water gone from the tubes, he can start breathing again and slowly comes back to normal! Cool, huh?