One of the most common questions I hear (granted this may be because I am the father of 3 children under the age of 8), is "Why do I have to poop right after I eat? Is the food I'm eating just falling out my butt?"
I also see the manifestation of this question not infrequently in my work as a physician when people say everything they eat just "goes right through [them]."
That isn't anything at all like what is actually happening:
When you eat, a neurohormonal axis is activated by the stretch in the gastric musculature, resulting in increased muscular contractions in the distal colon, and less so in the small intestine, causing movement of stool toward the distal rectum and anus, resulting in the urge to defecate.
In short, when you put something in the top end, your body tries to make room by moving stuff out the bottom end 😁