Those Transportation Security Administration requirements are drilled into every frequent flyer’s head: You can carry on liquids that are only less than 3.4 ounces (100 milliliters) in volume each. But when the TSA recently confiscated a jar of Jif under this rule, peanut butter lovers were up in arms. Some skeptics of security may suspect hungry officers just wanted to make their own PB&Js. TSA, however, contends that peanut butter is a liquid – and a full-size jar of Jif is over the 3.4-ounce limit.

Just like Americans’ favorite legume-based sandwich ingredient, the story – and the outrage it inspired – began to spread. However, Iowa State University Professor of Thermal Science Ted Heindel is a mechanical engineer who studies fluid flows, and the TSA action makes sense to him. By the scientific definition, peanut butter is indeed a liquid.

Read Heindel’s full article in The Conversation here.