Is it an exceptionally small cucumber or the world’s cutest watermelon? That’s the vexing question on the minds of visitors to the Windfall Farm stand at the Union Square Greenmarket these days. The Lilliputian specimens, each about the size and shape of a caperberry, have distinctive striped and mottled dark-and-light-green markings and taste like tart, lemony cucumbers with a not-off-putting note of watermelon rind. That cucumbers and watermelons belong to the same botanical family didn’t make solving the mystery any easier. Modern chef Gabriel Kreuther, who uses them as a garnish for chilled cucumber soup with lobster, is one slightly perplexed fan. “I’ve seen cornichon-size cucumbers before, but nothing like that,” he says. “I’m not completely sure if it is a true cucumber. I’d really like to know.” Although Windfall’s Morse Pitts had been labeling them cucumbers at his farm stand, even he admits he didn’t know exactly what they were or where they came from at first. After a little investigating, though, he discovered that a rogue farmworker had sprinkled a few seeds of a recently rediscovered heirloom variety called the “Mexican Sour Gherkin” and, thus, the case was closed. As the little cukes go for a cool $24 a pound and sell out faster than Alphonso mangoes, that farmhand might be due for a raise.