in ben radford and joe nickell's book "lake monster mysteries", the authors attribute a vast number of sightings to otter misidentifications. ed grabianowski plotted the distribution of north american lake monster sightings. then he overlaid the distribution of the common otter and found a near perfect match. it turns out that three or four otters swimming in a line look remarkably like a serpentine, humped creature undulating through the water, very easy to mistake for a single creature if you see them from a distance. "this isn't speculation. i'm not making this up," nickell said. "i've spoken to people who saw what they thought was a lake monster, got closer and discovered it was actually a line of otters. that really happens." clearly, not every lake monster sighting can be accounted for with otters, but it's an excellent example of how our perceptions can be fooled.