Hydro Flask's 21-oz. is the perfect go-anywhere companion on your busiest days. It stashes easily in a small shoulder bag, your pack's side pocket, or your car cup holder, and holds enough to keep you going for hours without weighing you down. The 21-oz. Standard Mouth, from Eastern Mountain Sports, is one of Hydro Flask's bestselling vacuum-insulated water bottles because of its size and versatility.