system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can I take a precautionary pre-movie pee that has a volume of 5 liters, but then have to race to the bathroom for fear of peeing my pants for  a thimble of urine?
gpt
You body is able to feel when your bladder (where your pee is held inside of you) is full, however that sense of how full it is, is not perfect. And just that feeling of having a full bladder is not enough to make you want to pee, really it all goes on inside of your head. From your past experiences you know certain situations that make you want to pee and you have a rough idea of how much you have drank and how much you probably need to pee. Have you ever heard running water and suddenly had to pee? Sometimes just thinking that you have to pee is enough to trigger that switch in your brain that says "its pee time!" So you can see that your brain controls the urge to pee, and certain things can make you want to pee more, if you have a lot of associations in your mind between watching movies and having to pee really bad, your brain will start thinking about peeing when you see a movie and then you'll have to go. Sometimes, you can just sit there for a second and focus on your body and how full your bladder actually is, this way you can counter the mental forces telling you that you have to pee, your body can actually hold it pretty long, it just gets uncomfortable (and it does get dangerous if you hold it too long, so don't force yourself to wait if you are feeling physical discomfort). 

One thing that makes it hard for your body to actually predict when you have to go pee is that your body holds a lot of fluids that aren't in the bladder as well and those fluids might make their way to the bladder at any time. Sometimes you will get the feeling that you really have to go, but not have much in your bladder, your body is just anticipating putting more in your bladder and wants to make sure there is room.