system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why do muscles spasm or twitch?
gpt
Your muscles are like electric motors.  They are controlled by nerves, which are just like an electric cord you see attached to your living room lamp.  

Sometimes the chemicals in your body are not balanced well, especially your potassium and salt.  Too much salty food or too much sweating can cause this problem.  Other times your body needs to repair or replace a cell in your nerves.  They do grow old and die sometimes.  

In each of these situations, the electrical signal that normally goes from your brain to your muscle is being interrupted.  Just like if you were to unplug your lamp and wiggle the tip of the cord in the electric socket, the same interruption is happening somewhere in your nerve.  This makes the lamp flicker the same way your muscles "flicker" on and off.  

A "loose connection" in your nerve causes "sparks" to travel down to the end.  The sparks cause the muscle to twitch or spasm.  
Until your body repairs the loose connection, or balances your chemicals in that section of your nerve, you can expect to continue to have the unwanted movement.