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)
