I got a useful natural history metaphor from my friend Jon.
There's a flock of birds grazing on the ground. These birds are prey to hawks, so some of the birds need to watch the sky. When a bird sees a hawk, it alerts the rest of the flock, and they scatter. If a bird watches the sky, it can't look for seeds or bugs, so sentry duty needs to rotate. If a bird neglects sentry duty, it prospers at the expense of the flock. (Well fed birds have a breeding advantage over dutiful but hungry sentries. On the other hand, a flock of non-sentries will be at a disadvantage vis a vis the hawks, and disappear.)
Birds can either look at the ground, the sky, or other birds.