He may do so in this way:

He will sit down and think about his problem (wanting more berries), and at the same time, think about specific alternatives for getting more berries.

In considering the mechanical effort needed to harvest berries, he will then imagine or conceptualize a tool that will decrease his effort and increase his production of berries.

He will think about a stick, let us suppose, and imagine how it might help him by dislodging more berries in less time, or in extending his reach to heights formerly above his grasp.

If he decides that he wants this stick badly enough, he well then go about finding a tree branch that can be fashioned into a stick.  He finds such a branch, breaks it from the tree, tears off the leaves, bark, and twigs, and soon he has himself a "tool".

What this individual has done is this:

He has made an "investment".  He has invested his time and energy by first applying his mental faculty to a problem, and envisioning a solution.  Next, he put form to his solution by searching out a suitable natural resource (the branch) and modifying it to his purpose and needs (stripping it of bark, leaves, and twigs).

An investment involves the foregoing of an immediate act of consumption (eating berries), in order to provide a device which it is anticipated will lead to a greater amount of consumption in the future.

When our primitive friend decided to visualize and then fashion a stick, he gave up a certain amount of consumption.

When he was working on obtaining and modifying the stick, he could not be eating berries, or resting in the sun, or enjoying any other act of consumption.