The phenomenon of twin babies appearing to communicate in their own “secret language” has long captured the imagination of both parents and researchers. Though many viral videos suggest a unique bond among twins—especially identical ones—experts clarify that true cases of a private language, known as idioglossia or cryptophasia, are actually quite rare and typically emerge only in extreme isolation. What’s more common is twins mimicking each other’s babbles, which is a natural and healthy part of early language development as their brains form speech connections.

One of the most beloved examples of this is a 2011 viral video featuring fraternal twins Merle and Stijn from the Netherlands. In the clip, Merle, dressed in pink, and her brother Stijn appear to be locked in a lively “conversation,” exchanging expressive babbles and intense eye contact. Their father, Roy Johannik, captured the heart-melting moment, which resonated with viewers around the world. Merle’s wide smile in response to her brother’s sounds only added to the charm, helping the video garner over 22