The British playground usually echoes to screechy impersonations of well-known artists such as Katy Perry. But if you approach one today you may well hear a seven-year-old adopt a vaguely Slavic accent and drone: “Sigma boy, sigma boy.”The lyrics come from an electropop track sung by two Russian tweens. The music video has racked up more than 271 million streams since being uploaded to YouTube in April. Academics are puzzled, but Sigma Boy has spread among tweens on social media faster than nits. The melody is repetitive, and the Russian lyrics – “I’m the icon of your icons / They dream about me as if I were bitcoin, got it?” – are unintelligible even in translation.