The popular ‘mermaid or fish / donkey or seal’ optical illusion has gone viral on TikTok, Twitter and stumped celebrities like JK Rowling – here’s whyOptical illusions are known to often break the internet and go viral on social media platforms like TikTok and Twitter, with a unique ability to divide opinion and create a conversation online. Popular optical illusions of recent years, such as the blue or gold dress debacle, have moved on slightly from more traditional swirling, kaleidoscopic and geometric patterns that play havoc with our visual perception by overstimulating the brain.