Bacteria growing in a bottle
This little exercise gives you a feel for exponential growth and what it means to resource availability. To get the basic idea try this.
Given: a bacteria in a bottle of substrate fills the bottle in one hour, starting from one bacteria.
The bacteria divides every minute.
Calculate: how full is the bottle at 59 minutes?
Discuss: with fast growing populations like some cities, or indeed the human population, at what point in the growth curve should the risk of running out of resources be addressed. And how should it be addressed?