Spoiler: It’s solid.
So determined Chester County President Judge James P. MacElree. In order to determine whether a thrown slice of pizza counted as a "solid object" under the law, His Honor undertook scientific observation of a pie:
MacElree said that to confirm his assessment, he bought a pizza and tested its physical properties. Unlike a gas, the pizza did not take up the shape or volume of the container in which it came, i.e. a cardboard box. When he sliced it into six pieces -- ''because I was not hungry enough to eat eight pieces," he said in an aside - the slices did not reform to take the shape of the container, and thus it was not a liquid.
To confirm that a slice was a solid, MacElree said he picked one up and noted that it retained its basic shape, "although it did droop a bit at the end.
"Further, I was able to bite off one piece which required some chewing before I could swallow it," he continued. "I put the remainder on top of a paper towel and observed that it stayed in place, did not spill onto my desk, and held its shape (less one bite)," he wrote. "I therefore concluded it was a solid."
I’m a bit disappointed that Judge MacElree did not rule out plasma or Bose-Einstein condensate.