Wildlife biologists use a method called capture / recapture to estimate populations of animals in the wild. For example, if a biologist wants to know how many deer are in a particular forest, or how many trout occupy a certain pond, it is impossible to capture and count all of them. Capture / recapture is a method which involves capturing a certain number of animals, tagging (or marking) them in some fashion, releasing them back into the larger population -- and then taking repeated periodic samples from the population to count how many of the tagged animals are recaptured in the sample groups. Mathematical proportions can then be used to calculate the estimated number of animals in the larger population.
That's exactly what we did in class -- we introduced a "tagged" sample of multi-colored goldfish crackers into a "pond" of the more common goldfish crackers. Then we recaptured samples from the ponds and counted how many tagged fish there were in each sample. We used those samples to calculate estimated numbers of fish in our ponds... It worked pretty well (except in a few cases where some of our samplers ATE the fish!!!). EEEEEKKKKK!