How can an ox ever become a cow? A fool's efforts are always in vain and yield no results.

This couplet wisely points out that you cannot change something's fundamental nature. Just like a bull can never become a cow, some things are simply impossible to alter. A 'fool,' in this context, is someone who keeps trying to achieve the unachievable, wasting their precious time and energy on vain attempts. It teaches us to recognize limitations and focus our efforts on what is possible and meaningful, rather than chasing after what can never be. It's a gentle reminder to understand the inherent nature of things and not expend effort where it will never yield results.