Remove a stack of four nickels from your pocket and place them on the back of your hand.
Cover the nickels with a small magical brass cover.
When the cover is lifted, the four nickels have magically changed into four dimes!
This incredible baffler is easy to perform, amazing to witness, and will leave your audience speechless! Loftus Version
Note: If the device over time with use becomes loose, just use a little sticky tack or magicians wax on the top. The gimmick will stay in place perfect.
Also see 25 Tips and Tricks with the Nickels to Dimes.