The Confusing Way The TSA Categorizes Peanut Butter - Tasting Table
Briefly

Packing carry-on bags can be challenging, especially with snacks. TSA classifies peanut butter as a liquid due to its ability to change shape, which means it falls under the 3-1-1 Liquids Rule allowing only 3.4 ounces in a quart-sized bag. However, peanut butter can be packed in checked luggage without limits, and spreading it on a sandwich allows travelers to carry it on. Other surprising foods, like icing on cupcakes, are also classified as liquids, leading to confusion at security.
In 2012, TSA agents at Las Vegas International Airport confiscated a woman's cupcake. This cupcake in question was a red velvet cupcake stored in a Mason jar with a thick layer of icing. The icing, not unlike peanut butter, conformed to the shape of the Mason jar, which in the eyes of the TSA classified it as a liquid, prompting the confiscation.
The TSA classifies peanut butter as a liquid due to its non-definite shape, meaning it falls under the strict 3-1-1 Liquids Rule for air travel.
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