Penny Polisher by Wunderground Magic

The magician (that's you!) shows an old penny and offers to shine it up with a magical new penny polisher. Suiting actions to words, the penny is polished up...but now it's a shiny dime! Showing a second penny, you offer to show what happens when you polish for too long. After waiting 5 seconds (a long time for people these days) the penny is shown to have shrunk down to a tiny little mini penny! Very easy to do, complete with two magic penny gimmicks, a mini penny, and written instructions.