Step 1: Nitrocellulose
Nitrocellulose is a plastic like substance, used as a wood varnish, fingernail polish, guitar picks, ping pong balls, and in the manufacture of many other useful products. In most instances nitrocellulose burns quite slowly in open air, but when in the low density form of flash cotton, it burns quickly and cleanly enough that the heat rapidly disperses into the air without burning the surface it is ignited on. This means that if done properly, small amounts of it can be lit on top of an open hand without causing a burn. This is not a recommended activity to try, as burns are of course a possibility.
In the above video I ignite far more flash cotton in my hands than what would be considered safe. I have years of experience working with nitrocellulose, and know exactly what the necessary precautions are for working with such large amounts. Under no circumstances should my actions be replicated in that respect.
Step 2: Trapping the Ash Tray
This prank is sure to get a big reaction, so I highly suggest that it not be used on someone who's reaction may not be a positive one. Use common sense, and don't prank in public places.