Show people a rounded blob and a spiky star and ask which one is 'bouba': most pick the round one. A 2022 study of speakers of 25 languages, using very different writing systems, found the effect in most of them. The link between sound and meaning, it turns out, isn't entirely arbitrary.
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.