While blowing your nose, a booger that gets stuck and resonates the inside of your nasal passage and causes your eyes to water. This makes the false appearance that you are crying.
Greg walks into Jeff's office
Greg: Hey Jeff I have those proposals you asked for, oh shit are crying?
Jeff: No man, I was blowing my nose and came across a wicked cry booger.
Greg: Ahhhhhhhh, I see..