Final answer:
The word 'observation' in this context means carefully watching and studying something, as Paul and Steve did with the globe and comparing geographical sizes.
Step-by-step explanation:
In the context of the text provided, the word "observation" refers to the act of carefully watching and studying something. Here, Paul and Steve are described as engaging in 'after-school observations,' which implies that they were examining and learning about the globe and using it to make comparisons with the world's geography, such as the size of the United States relative to the Sahara Desert.