Functions, first as a normal bag to carry any personal belongings, second, you can install the water bag to use as a hydration bag.-PROFESSIONAL 3L HYDRATION BLADDER: The 3L liner inside detaches from the backpack for easy cleaning and assembly. The rubber mouthpiece has a simple switch valve to stop water flow when needed, ensuring a leak-free outdoor experience. Our water liners have passed pressure tolerated tests and have been put to the test to the highest quality standards.-The bag with a 100cm length water pipe can be easily placed in the bag for you to drink water with compressive resistance to abrasion.-Tube connected to the bottom of the bladde