You look cautiously into the hollow concrete structure and quickly conclude from the sewer grating barring you from the clouds above and from a swiftly running underground river below that the structure is some kind of drain for the empty canyon that must sometimes be a riverbed.  “That’s cool,” you think noncommittally, “maybe I can hide out here for a little bit.”  Either way, you know you can’t stay right where you are; your head in the drain and your butt pointing skyward.  You look back and don’t see or hear any sign of someone following you, and you are pretty tired from all that running.  You


Stand up and keep running; no time for rest!