system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do our tummies rumble when we are hungry?
gpt
This can be explained by a closer look at how the digestive system functions.

The digestive system is, in essence, a long tube that starts at the mouth and ends at the anus. This tube connects with the various organs and passages that play important roles in digestion. One of the most important things to know about the digestive system is the manner in which it propels food. Waves of muscle contractions move and push the contents continually downward in a process called peristalsis. In addition to moving your meal along its digestive path, these contractions also help churn food, liquid and different digestive juices together, rendering them into a gooey mix known as chyme.

Stomach growling is the result of this process. Moving with those solid and liquid chyme ingredients are gasses and air. As all these ingredients get pushed around and broken down into easy-to-absorb bits, pockets of air and gas also get squeezed and create the noises we hear. Stomach growling can happen at any time -- not just when you're hungry -- but if there's food in your stomach or small intestine, the growling becomes quieter. It's like putting a pair of sneakers in the dryer by themselves versus with a load of towels. The towels muffle the noise of the shoes as they bounce around.

But you may be wondering -- if your stomach is empty, why are the muscle contractions that digest food happening to begin with? The reason has to do with hunger and appetite. About two hours after your stomach empties itself, it begins to produce hormones that stimulate local nerves to send a message to the brain. The brain replies by signaling for the digestive muscles to restart the process of peristalsis. Two results occur: First, the contractions sweep up any remaining food that was missed the first time around. Second, the vibrations of an empty stomach make you hungry. Muscle contractions will come and go about every hour, generally lasting 10 to 20 minutes, until you eat again.