A more thorough explanation:

Definition: Life in being refers to the concept under the rule against perpetuities that anyone who is alive when a future interest is created, regardless of whether they have an interest in the estate or not. This means that the future interest must vest within the lifetime of a person who is alive at the time the interest is created.

Example: Let's say that John creates a trust that will transfer ownership of his property to his grandchildren 50 years after his death. However, the rule against perpetuities requires that the future interest must vest within the lifetime of a person who is alive at the time the interest is created. Therefore, if John's youngest grandchild is currently 10 years old, the trust would violate the rule against perpetuities because it is possible that the youngest grandchild could live beyond 60 years after John's death, which is the maximum time allowed under the rule.

Explanation: The example illustrates how the concept of life in being applies to the rule against perpetuities. In this case, the future interest in the trust must vest within the lifetime of the youngest grandchild who is alive at the time the interest is created. If there is a possibility that the interest could vest beyond the lifetime of that grandchild, then the trust would violate the rule against perpetuities.