Looking for a quick effect for the Trick or Treaters? How about a effect where whenever any ghoul and goblin makes it to your door a glowing ghostly image appears out of nowhere?

All it takes is some liquid laundry detergent (cheap as free), a blacklight bulb in a socket on a cord ($15 for a compact florescent and lamp), and a sensor plug ($20).

1) Tide, Woolite and other liquid detergents glow under blacklight, but are clear in normal light. Paint it on thick and make a hidden masterpiece!
2) Hook your lamp with the blacklight into the sensor plug and stick in in the socket on your porch.
3) That's it!

The sensor plug flicks whatever is plugged in on when it senses motion, the light switches on and your art glows! The light itself coming on with nobody right there is a surprise, but the picture makes it.

... Sorry for the poor photos, need to touch up the art and photograph then. Can you guess who from the poor picture?