In Which She Shows

Lisa observed that their expressions were identically blank and perplexed whenever either one of them was asked anything. 

This was the case regardless of which of them was being questioned. When she compared the two of them, she noticed that there was a difference in this regard.

During the course of Lisa's investigation into the parallels and divergences that exist between the two, she made the following discovery. 

During the course of Lisa's inquiry into the similarities and differences that exist between the two, she came upon a facet of the scenario that was particularly relevant to the subject matter under consideration.

When Lisa noticed that the two of them seemed to have expressions that were the same whenever they were asked anything, she bit her lower lip tightly together in an effort to suppress a giggle. 

She noticed that the two of them seemed to have expressions that were the same whenever they were asked anything.