This phenomenon occurs when one is in the process of defecating. In order to occur, the fecal flux must be a single, uninterrupted outflow, creating a log of such magnitude that the stool reaches the bottom of the bowl before being released. Once this delicate equilibrium is broken, the crap proceeds to fall forwards, gently tickling the underside of your testicular sac, leaving a brown deposit in its wake. This rare occurrence is known as the "brown bag".
-dude, your toilet bowl is so shallow, it gives me brown bags all the time!!
This phenomenon occurs when one is defecating in an unclean and/or unsafe environment, and does not wish to make physical contact with the toilet seat. As a result, the person performs the squat maneuver. This accomplishes two things: first, the point of exit for the stool is at a considerably higher elevation, resulting in a high entry velocity. Second, as a result of the pivot point being above the feet, the point of exit is further forward in relation to the bowl. This point of release may be so far forward such that there is no (or shallow) water directly above the asshole. If the consistency of the crap is just right, it will land and stick, remaining in an upright position. If the center of mass is directly above the area spanned by the base, the log will be in metastable static equilibrium, and will not fall down. This is commonly known as the lighthouse effect.
Note: the lighthouse is said to have weathered the storm if it remains even after the ensuing flush, such that there is no toilet paper or urine detracting from the natural beauty of the phenomenon.
-Dude, I just left a lighthouse in that bathroom!!
-Nice! Did it weather the storm?
-Yea! Come check it out!