In a prior study, we investigated graduate student reasoning on a set of entropy-related conceptual tasks in a think-aloud format. The tasks involved entropy from microscopic and macroscopic perspectives, ideal gases, and a novel context involving a system with a dynamic string. In the current study, we conducted interviews with undergraduates using the same questionnaire. Most students were interviewed during the second half of their upper-division Thermal Physics course at the University of Colorado Boulder while two were upper-division undergraduates from other institutions with strong physics programs. We analyze the responses of the undergraduates to a section of the interview involving a novel system of a dynamic string waving in a bath of water and discuss the similarities and differences between the undergraduate and graduate students' responses. The responses from the two populations share many similarities with a few noteworthy exceptions. The undergraduates generally did not produce multiple macrostate classifications in the novel system, and some expressed a concern with what they perceived to be an infinite number of microstates---and thus infinite entropy---which was not a concern among the graduate students.