Impromptu magic always has a huge impact - this is perfect for those impromptu situations.  Magician takes a coin and proceeds to apparently bite a piece from it.  It's close as you like, yet there is clearly a piece of the coin missing!  The magician then proceeds to return the piece from the mouth onto the coin, instantly it re-appears and the coin is complete once more

Made popular by David Blaine in his Street Magic TV specials.  The Magician takes an ordinary looking coin and proceeds to bite off a piece.  Displaying the coin with missing bit, the magician now apparently spits the bitten piece back onto the coin, where it visibly restores.  

Available as either 10p or 50p British currency.  Before purchasing this trick, you are advised to learn a coin switch (Bobo switch or similar are ideal), purchase of Bobo Modern coin magic book or DVD or Dave Jones Beginners Coin Magic DVD are suitable.