Fake Gum
For this simple instructable you will need card stock, scissors, aluminum foil and about five minutes. Trust me, it's totally worth it.
Cut a piece of card stock that is about as big as one of those long, thin Wrigley's gum pieces. Then cut a piece of aluminum foil that is about 3x the whith and 1 1/2x the hight of the piece of card stock. MAKE SURE THE CARD STOCK AND ALUMINUM FOIL ARE BOTH CUT STRAIGHT! Your "gum" will look a lot more believable this way.
Place the card stock in the middle of the aluminum foil. Fold the sides of the aluminum foil to the middle. Then, fold the top and bottom of the aluminum foil like you would with the first step to folding a paper airplane. Finally, tape the tops to the card stock part. Make sure all of this is folded very tightly.
Now it's time to pull the prank! Go find your victim and offer him or her "A piece of gum". Your victim will be disappointed to find a piece of card stock instead of gum. P.S. If you want the prank to be even worse, you can replace the card stock with ABC (already been chewed) gum. If you want to do this, I would suggest still using card stock to make sure that the gum is not all floppy.