Being in medical school, I have friends and acquaintences asking me for medical advice all the time. This was particularly disconcerting when I was a first year student and knew little to nothing about medicine. As a third year student, I feel much more comfortable answering these questions because I have learned so much over the previous two years and I have a much better idea of what I don't know, which means I have a better idea about when I should recommend that the friend sees their physician. Most of the questions I get asked start with "I have this rash..." or "I've been feeling under the weather..." but every once in a while I get a more interesting question.
A friend of mine was watching a movie and had to urinate causing him to miss an important scene. Never wanting to go through this again, he wanted help in developing a strategy to avoid a similarly horrendous experience in the future. His one caveat was that he didn't want to give up his 2L pop while at the theatre (yes, drinking 2L of pop at a go is unhealthy, he doesn't care, move on...). One idea was self cathetrization, but he didn't really like the idea of inserting a tube into his urethra (there is also a significantly increased risk of urinary tract infections, so don't do this...).
Alternatively, he could just make sure he ate salted popcorn while at the theatre. How does salted popcorn help? Well, he had to urinate during the movie because he was consuming 2L of fluid and his kidneys were flushing this extra fluid from his body. However, if he ate salted popcorn, this would increase the concentration of sodium in his blood. To bring the concentration of sodium back down to normal levels, his kidneys would initially have to retain fluid...thereby reducing the amount of urine they produce. Over the next few days his kidneys would excrete the excess sodium and the fluid that was retained with it to bring his body back to normal fluid volume and sodium concentration. However, in the short term, producing less urine while watching the movie means that he would not have to pee until the movie is over.
Of course, there are a lot of reasons why consuming excess salt is a bad idea, but this friend is a healthy guy, so his body can take a short term excess sodium load. So this idea will work well for him.