If you learn something about whether it’s raining, from some source other than observing the sidewalk to be wet, this will send a forward-message from Rain to Sidewalk wet and raise our expectation of the sidewalk being wet. If you observe the sidewalk to be wet, this sends a backward-message to our belief that it is raining, and this message propagates from Rain to all neighboring nodes except the Sidewalk wet node. We count each piece of evidence exactly once; no update message ever “bounces” back and forth.
