It’s deceptive because it’s actually one of those non-canning Ball jars that’re made for decoration or drinking from, and the coloring on the outside is peeling off. The liquid is just FFJ I’d made from a melon that sat too long and went squishy.
It had a fizzy scum layer on top, and I gave it a light stir to measure out a bit to use, and it started fizzing up a couple of seconds later and overflowed!