Kai, like most boys aged around 18 months and up, obsesses over large wheeled machines. Thomas the Tank Engine, Hotwheels cars, Tonka trucks . . . he can hardly get enough of them. Why? It won't surprise you to hear that I have a hypothesis (not a full-blown theory) about that.
Assume, as seems likely, that humans evolved in an environment that favored males skilled in fending off, hunting, or herding large fauna. Assume further, as also seems likely, that males needed to learn those skills. Boys were not born knowing how to lure a cave bear into a killing pit, in other words; they had to study and practice such things. Absent genetic transmission of those animal management skills, evolution selected the next best thing: An innate childhood fascination with animals.
A young human male might thus end up with an inborn disposition to practice watching and manipulating models of the mobile, animal-sized things he observes around him. Back on the prehistoric savanna, boys might have played with wee clay antelopes and lions. Their modern counterparts, surrounded by machines rather than animals, favor toy trains, cars, and trucks.
The behavior of Kai and many, many other boys conforms to that explanation. So, too, does observation of the many near misses. Clouds, rivers, and rain exist in nature and move, yet they don't inspire boys' fascination. Boys also tend to ignore machines that don't move, such as copiers or electrical generators. Perhaps because no ancestor won the evolutionary game by hunting mice or herding bugs, models of small mobile machines, such as scooters or lawn mowers, don't score big points among young males. For similar reasons, toys based on boats and planes come in a distant second to their land-based counterparts.