Shocking gum is a practical joke

Practical joke

A practical joke is a mischievous trick played on someone, typically causing the victim to experience embarrassment, indignity, or discomfort. Practical jokes differ from confidence tricks in that the victim finds out, or is let in on the joke, rather than being fooled into handing over money or...

 that gives a harmless electric shock

Electric shock

Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....

to the target. The target pulls a piece of gum which is pushed forward a little in a 5-gum box. The shock does not harm the target but it annoys them. A few novelty companies in the 1950's produced these 'Shocking gum' packages.
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The Popular Brands of Shocking Gum
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• Fruit Juicy
• JB