system
Given the following post, respond as if you're commenting on Reddit:
human
When I pee, why does it feel near impossible to simply stop peeing half way through and just finish the rest some other time?
gpt
You've got a muscle that controls your bladder, and to urinate you need to relax it. 

Imagine you're arm wrestling with someone. They're applying a pressure on your arm, and your arm muscle is resisting that pressure. Now, if you relax your arm, they'll push it onto the table, and it's really hard to recover and force your arm back up. You need to be really strong to recover from that position. It's easier if they let go.

The same thing applies with the muscle controlling your bladder. You can train it to shut off the flow off urine, but it needs to be strong enough. The stronger this muscle is, the easier it will be to stop urinating. When all your urine is gone, there's no more pressure, and your bladder muscle can seal up.