When you're trying to determine whether a rule is always true or false, sometimes it helps to think about really extreme cases, e.g.:

  • when considering polygons, consider the fewest number of sides (triangle), and polygons with thousands or millions of sides

  • when considering angles, be sure to consider 0 and 180 degrees (or angles very close to those)

  • when considering lengths, be sure to consider 0 and improbably large values (e.g., a billion kilometers)