We overdo it at the gym and end up sore the next day. As we get older, parts of our body that were unprovoked begin to nag at us. Our knees get stiff and we can't get up as fast as we used to. Older still, our bodies begin to seem no longer our own. We cannot walk as fast or run as far.
Stairs suddenly seem twice as long.
As with any complex machinery, especially if we have not taken good care of our bodies, or have just outright abused them, they will begin to fail us. There are also the discomforts of illness and disease. We may suffer from stress and get ulcers. We may abuse alcohol or drugs that damage our bodies inside and out. Our lungs may become blackened from excess cigarettes, cigars, or marijuana.