How can a shared resource be managed across rounds?

When several people use the same resource, each person's use changes what is left for everyone. Following that remaining amount, called the stock, helps explain why a shared resource needs attention across time rather than only during one person's visit.

An invented pond begins with twelve fish tokens. Three users each request two, so six tokens are removed and six remain. Under the stated renewal rule, three tokens are then added. Nine become the starting stock for the next round, fewer than the original twelve.

Taking happens before renewal. Removal cannot exceed the stock already present, and renewal cannot raise the stock above the pond's token capacity. In this illustration, no tokens are added if none remain. These are declared teaching assumptions, not biological rules for how real fish populations grow.

Users can agree on limits and keep track of the remaining stock because today's removals affect tomorrow's starting point. Different resources need different arrangements. The example explains that connection without claiming that depletion is inevitable or that one rule works for every shared resource.