You know how uncomfortable it feels when you really have to go to the bathroom?

当你十分想去厕所的时候，你知道那种感觉有多痛苦吗？

That means on a plane, after you pass security, you should purchase extra water and drink enough of it on the flight so that you have to go to the bathroom to urinate.

也就是说，在通过安检登机后，要额外买水并饮用足够多的水，以入厕小便。