Here's why:

* Cycles in nature are closed loops. This means that the substances involved are constantly being recycled and reused. There's no true "end product" in a cycle, only transformations.

* Water is a key component of many cycles. For example, in the water cycle, water is constantly evaporating, condensing, and precipitating. It's not an end product, but a driving force.

* Water can be a byproduct of reactions. For instance, in cellular respiration, water is produced as a byproduct of glucose breakdown. However, this water is then used in other biological processes, making it part of a larger cycle.

Therefore, there is no specific cycle where water is the final, unchanging product.