How does the author develop the theme that obligation is what distinguishes work from play?

In context, we might look at Tom's punishment... Aunt Polly telling him to white-wash the fence. For Tom, his obligation is work... so in order to coerce the other boys into paying to take part in his punishment, he makes it look like fun, and for the boys.... it is. Obligation is work.... the task, when seen from a different perspective, is fun.