What literary device is illustrated by the phrase "the curb is a tightrope"?

Read the excerpt from "object lesson, part 2." "no sign of the original envelope? " "no." "i found two boys with bills — in each case a single, too. david strager and joey buell. no envelope." louise's brows met. ellery glanced up at the clock. 9: 07. he strolled over to her. "don't show them you're worried. there's nothing to worry about. we have twenty-eight minutes." he raised his voice, smiling. which statement explains the inference ellery makes and why he makes it? he infers that miss carpenter thinks they are not going to find the thief because he sees her facial expression. he infers that it will take twenty-eight more minutes to find the missing money because that is all the time he has. he infers that the original envelope will not be found because neither david strager nor joey buell has it. he infers that david strager and joey buell took the money because they each have a dollar bill.