The most difficult aspect of the rule against perpetuities is figuring out a sure-fire way to determine whether an interest created in a conveyance is valid or invalid. The meaning of the rule itself is not hard to fathom. Whenever the interest might vest too remotely it is invalid, and it becomes possible to vest remotely if there is a chance that it could vest more than twenty-one years after everyone alive at the time of the conveyance has died. Whether the interest violates the rule against perpetuities is determined at the moment the conveyance creating the interest becomes effective. The problem is to demonstrate that, at the time of the conveyance, there exists or does not exist at least one possible future situation in which the interest could vest too remotely. It is a problem that has caused many a law student more than a few "Gray" hairs.
How to Do a Perpetuities Problem,
36 Clev. St. L. Rev.
available at http://engagedscholarship.csuohio.edu/clevstlrev/vol36/iss1/12