system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do injuries hurt when weather changes occur?
gpt
The most popular theory blames the drop in atmospheric pressure that accompanies rain. Think of it like blowing up a balloon. There’s inside air pushing out, and outside air pushing in. Atmospheric pressure is always pushing on our bodies in a similar way. If the pressure outside of the body drops, gasses inside the body can expand, particularly the gasses dissolved in the fluid surrounding your joints and tendons.

Now consider your injury. Think about pain as nerves that have been irritated. Somehow they didn’t heal right, or they might be inflamed. That expansion of fluid around your damaged Achilles might be microscopic, but it can really trigger these nerve endings by putting additional pressure on them, causing pain.

As for the temperature, the theory goes like this: cold may cause muscles to tense up, and that contraction can also aggravate those damaged nerves.

Long story short: A drop in atmospheric pressure may trigger an increase in the pressure of your body’s fluids on damaged nerves, causing pain.