A structure is a body part, and a function is the action that a body part performs. The effects of biological adaptation can be studied by investigating structure-function relationships. For example, caterpillars feed on leaves; therefore, their mouths are shaped to facilitate chewing. Although this biological adaptation maximizes eating efficiency in the larval stage, the same mouthparts in an adult monarch would be useless. The butterflies have a proboscis, which is a long, straw shaped mouthpart, used for sucking nectar. Students can observe the different structures found on the organisms, as well as investigate the functions these structures afford.

Children observe a structure, such as the antenna or the prolegs, on the caterpillar and form conjectures as to what function the structure serves. For example, children may think the antennas on the back of the caterpillar are there as sensors, acting as a defense mechanism. Children also have questions about the anatomy and physiology of the caterpillars, which they may research. They wonder about digestive and circulatory systems in caterpillars and monarchs. Children begin to think about biological adaptations organisms have made due to their environment.