Talk:Sans/@comment-174.30.106.234-20151103182651/@comment-81.141.65.202-20151103202121

The general theory is that San's is/represents half of WD Gaster, with Papyrus being the other half. Hence only his left eye glows, we can assume if Papyrus ever got serious his right eye would glow.

The Gaster Blasters also have illuminated left eyes once they are fired.