An invariant is a quantity or property that remains constant throughout a series of transformations. This concept is widely utilized and studied across various disciplines, including discrete mathematics, which is a compulsory course for computer science majors.

A brain teaser related to invariants might not seem academic at first glance. Instead, it assesses the candidate's ability to first recognize that the problem can be solved by applying the concept of an invariant and then to identify this invariant.

The example below is a classic one. Such problems often present an infinite number of possible transformations. Many candidates might manage an educated guess to assert that the goal is unachievable, but they fail to provide clear and convincing reasoning.