Someone asked how to fill the backpack with 20 lb.

First - pack your bag with usual stuff you take to any hike. Water, food, jackets, first aid, ten essentials and any other gear or equipment. Think about things that you will need to take to a climb, and weigh the bag. I assume it must be between 10 - 15 lb. You do not have all the gear for now. So, you may not have 20 or 25 lb today. See how much you need to add to reach 20 or 25 lb. A liter of water will be 2.5 lb  approx (including weight of bottle). So, you can add one or two bottles of water. If you have a water bladder, fill it - if it is 2 liter or more, it will add enough weight.

If you add a milk gallon full of water, it will be difficult to make the weight symmetric, it may hurt your shoulder, except - if you add two of them, that may be needed if you need to have 40lb or more.

Always - start with filling the bag with actual gear, and add more gear to increase weight - things like micro spikes, additional gloves, and jackets etc. That is easy. Adding extra gear will help distribute the weight symmetrically. if you still cannot reach the required weight for the particular hike, then add water in bottles or cans.