The magician shows a penny and covers it with a block.  Instantly, the penny shrinks to less than half it's original size - into a miniature penny.  The block and mini coin can be immediately examined by the specatator and the secret will not be discovered.
This trick can also be used to change a penny into a dime - and again, all can be examined without fear of divulging the special secret working.

Video instructions included.

Comes with special block, special penny and miniature penny. (Use any regular dime.)