How do you understand that a mountain depends on water, but you can't do it yourself?

This sentence should not be the original words, the original words should be relying on the mountain to eat, relying on the sky to eat, people are old, but no matter which sentence, the meaning expressed is similar, just showing that people can't get what they want by external forces or others, because these people can't help you forever, or these external forces can help you forever.