They don`t have liquid walls on earth?! They use hard walls, so your actually gonna knock your yourself out if you try to walk through it. To solve this problem humans have invented doors. The doors are everywhere. They are so popular even a pop group is named up after them. The concept with the door is to provide an opening in the wall where humans can walk through without having to destroy the whole wall. It actually opens. Making a hole in the wall, that is open for a short period of time. In this time frame perople can walk through it. Usually this this time frame lasts for a few seconds before the door closed and becomes part of the wall again. Only selected people are allowed to enters doors. And this selection happens with keys. The people approved worthy of door entering gets a key who opens a lock in the selected door. This key givens them 24 hour access to this door. Popular doors have heavy locks, less popular doors have smaller locks and some doors don`t have locks at all. Usually cause their so unpopular nobody wants to got there. The really popular doors have armed guards to keep of all the people who wants to get in. Others have bouncers who charge money to get inn. And some are thrown into doors they don`t wanna walk through. Then there`s electric doors who doesn`t give shit who you are. There is a door for everyone.