Wrap a nickel-size chocolate coin in silver-colored gift wrap paper. Place the wrapped coin in the refrigerator freezer until you are ready to perform your trick -- then hide the coin in your left pocket.
Pour coffee and water into a coffee maker, or pour liquid coffee into a coffee cup and set the cup in front of a microwave oven. Fill the bucket with water and set the water bucket next to the coffee maker or microwave.
Stand before your audience. Tell your audience you are going to show them how a nickel can be melted using an ordinary cup of coffee.
Start the coffee maker, or put the cup of coffee in the microwave and heat the coffee on the highest setting for one minute. While making or heating up the coffee, entertain your audience with made-up tales of how wizards from India, China or medieval alchemy were able to melt coins using only hot water.
Ask a member of the audience for a nickel when the coffee is hot and ready, and invite the volunteer to stand up front with you. Hold the coin between the thumb and forefinger of your outstretched right arm, and move that arm back and forth so the audience can see the nickel; meanwhile, casually place your left hand in your left pocket and palm the chocolate coin. Invite the volunteer to examine the water bucket, distracting him (and the audience) while you are palming the coin.
Return the real nickel to the audience volunteer. Remove the hot cup of coffee from the microwave, or pour a cup of hot coffee into the cup from the coffee maker. Ask the volunteer to carefully check the coffee to see that it is hot, and then have him carefully drop the real nickel into the coffee cup.
Pick up the coffee cup with your right hand and, pronouncing out loud whatever magical incantations come to mind, pass your left hand over the coffee cup and surreptitiously drop in the chocolate coin. Continue your incantations for at least a minute so the chocolate will melt. Pick up and lift the water bucket high over your head -- so the audience and the volunteer can "see" the bucket well, without being able to see into it. Quickly pour the contents of the coffee cup, coins and all, into the water bucket.
Roll up your right sleeve, reach into the water bucket with your right arm, swirl the arm around and pick up the remains of the melted coin -- the gift wrap. Crumple the remains as you remove them from the bucket, to hide any traces of chocolate. Display the "melted coin" to the audience, then drop the gift wrap back into the bucket. Remember to hold the bucket as high as possible. Swirl your right arm in the bucket again, and draw out the real nickel, which you will then return to your flabbergasted volunteer.