This is a fermenting watermelon that's been sitting on our counter for a day or two. The foam is oozing out of a crack that appeared in the watermelon, leaving a puddle of rancid goo. I suppose we should have cracked it open and made an evening out of sipping watermelon wine but we decided to trash the whole mess instead. This was store-bought melon. Our garden melons are still growing on the vine!