I got serious about studying and performing magic when I was about 13 years old, but when I was seven or eight, I got an Adams magic set, which began a slow percolation of my lifelong love for magic. Among my favorite tricks in that set was the Adams "Magical Block" -- a red plastic block that could be placed on a penny, and when lifted the penny had changed to a dime (or vanished, but I liked to make it change to a dime). My Adams black was plastic, not the earlier wood version, but the penny gimmick was made from a "wheat back" penny, which were still fairly common in the 1960's when I got this trick.
Adams Magical Blocks can still be found, but usually collectors do not want to part with them. A number of versions of the effect have been made over the years, but most are not as sure working as the Adams original, and now almost all these versions come with a poorly reproduced fake penny gimmick. But I recently found this nicely made version, which works well, and with a great looking penny gimmick. (To be honest, I thought the gimmick was made from a real penny... but I can't tell for sure, I am saying this gimmick is a reproduction of a US cent. It may be real, but it doesn't really matter because it looks so good, I am okay to just tell you that it is a reproduction.)
This set also comes with a mini penny, so you can do any of three possible effects: vanish the penny, change it to a dime, or cause it to shrink to a mini penny. :-)
Enough preamble.... Here's the effect:
A black plastic block (decorated with a mystical star) is placed on a penny. When lifted,, the penny has shrunk to a mini penny. (Or vanished, or changed to a dime.)
The block and mini penny can be examined (with one simple secret move to ditch the gimmick).