For example, assume O conveys "to A for life, then to the first child of A to reach age 30." Assume that A is alive when the conveyance takes effect, but that A has never had children. A potential unborn child (A's first child to reach age 30) receives a contingent remainder under this language, which is a type of interest subject to the Rule. It cannot be logically proven that this interest is valid. For instance, A might have a child, B, one year after the conveyance; suppose O and A then die. Twenty-nine years later, if B survives, her contingent remainder will "vest" by becoming a present estate. B's interest is deemed invalid under the Rule—at the time of O's conveyance—because such vesting would come too late (more than 21 years after O and A, the lives in being, died).