This is Zipf's law: in a large body of text, the second most frequent word shows up about half as often as the first, the third about a third as often, and so on. It holds in every language studied. In English, 'the' alone makes up roughly 6 to 7% of written words. A handful of words do most of the work.
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.