If you’ve ever noticed an influx of mermaid-themed artwork posted online during May, there’s a good reason for it: MerMay, a challenge that encourages artists to draw a different mermaid each day of the month, with 31 prompts to help guide them along. The art challenge has officially been around since 2017, though the artist behind it had no idea it would become so big. Veteran Disney animator Tom Bancroft, the supervising animator for Mulan’s tiny red dragon Mushu, just decided to draw some mermaids one day. “It was by accident,” he tells Polygon. In 2016, he drew a picture of a few mermaids hanging out on rocks in the ocean, talking on their “shell phones.” (“Get it?” he adds.)