Lyric discussion by Kweder 

Cover art for Letterbox lyrics by They Might Be Giants

The "letterbox" is the future. The "sparrow" is time gone past (in the fourth dimensional, shouldn't I know better than that based on the things that I have learned, seen and done timeline that we all exist upon - and, as such, shouldn't I know better than all of you). We know and understand what has happened, yet the future does not seem to respect our predictions.

The song is about the fact that we all have lived our own crazy lives, and have seen the things that we have seen - we individually think we know what is really going on - we expect life going forward to congeal with what we think the future will be.

But of course, it doesn't. The future keeps confusing us, it acts like the ongoing culmination of the not so random sequence of events that it is going to. And still, we act suprised that that the ongoing result of the culmination of billions of people interacting with each other doesn't fit our own selfishly tailored view of what will happen tomorrow.

Despite our confusion, life goes on. And the mail will show up tomorrow.

Repeat.