A spectacular example of intelligence can be observed watching a squirrel hack his way into a bird feeder. The squirrel will use trial and error and systematically try every possible way of getting the free meal, until ultimately his curiosity and tenacity is rewarded.
We observe a similar behavior in mice navigating a maze. To navigate a maze is part instinct and part a learned response; instinct for food drives the mouse to forge ahead. While he learns the maze the mouse cycles back-and-forth to previous points in the maze trying and re-trying the different paths until ultimately he succeeds. The mouse internalizes the maze, remembering previous failures and then in anticipation of his reward he forges ahead.
Both of these mental processes, hacking the bird feeder and maze navigation, illustrate a dynamic mental model of intelligence, where complex actions are trialed and remembered; and then systematically modified to give rise to remarkable results. Our ancestors learned to apply our brains to solve the day-to-day problems of survival as well. We learned to use our instinct for curiosity to examine the world for opportunity. We then applied our tenacity to exploit that opportunity into something benefited our survival.
When we call intelligence in humans is just our more complex version of "hacking the birdfeeder". We internalize a problem and once the model of the outer world is internal, we are free to manipulate it in our mind and trial different scenarios first mentally, then physically. This manipulation of our subjective experience we call thinking, and when we successful use it to solve a problem, we call that intelligence. Just like squirrels and mice, those of us blessed with more intelligence have higher degrees of curiosity, tenacity and memory; which grants us higher success in applying this process.