In the 15th century, poet François Villon, convicted more than once, wrote several ballads 'en jargon', the secret language of the Coquillards. This gang of criminals was tried in Dijon in 1455, and the trial records preserved a list of their code words. Even today, scholars can't fully decipher these ballads: a slang designed not to be understood did its job.
In Kuşköy, a mountain village in northern Turkey, people talk across valleys by whistling Turkish. A 2015 study showed that whistled Turkish draws on both hemispheres fairly evenly, whereas spoken Turkish relies mostly on the left. The physical form of the signal, it turns out, matters to the brain.
Speech has no gaps between words. In 1996, researchers played 8-month-old babies a continuous stream of made-up syllables for just two minutes. The infants picked out the 'words' by tracking which syllables tend to follow one another. That's probability calculation, done without a calculator or any visible effort.