Foolproof, really easy and so impressive!
Take a clear salt shaker, full of salt, and sprinkle a few grains into your hand (even dump them on something dark to the spectators can see them). Make a fist, and turn the shaker upside down into your fist, letting all of the salt pour out of the shaker. (A few grains can even spill out of your hand.) Show that the shaker is clearly empty. Then open your fist- instead of a fistful of salt, your hand is EMPTY!
But wait- you are not finished yet! Place a silk or other cloth over the empty shaker for a few seconds (and maybe have the audience shout a magic word). Lift the cloth away- the shaker is full of salt again!
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