Gel is a fascinating material for its unique properties, such as phase-transition, chemomechanical behavior, stimuli-responsiveness, low surface sliding friction, and for its possible wide application in many industry fields. Recently, hydrogels have drawn special attraction in biological field due to its possible applications as soft man-made tissues. However, conventional hydrogels, especially polyelectrolyte gels, are mechanically too weak to be practically used in any stress or strain bearing applications. Inspired by the structure of articular cartilage, we discovered a general method to obtain very strong polyelectrolyte hydrogels containing 60-90% water by inducing a double-network (DN) structure for various combinations of hydrophilic polymers. The soft and wet gel materials with both a high strength and an extremely low surface friction would find wide applications not only in industry but also in biomedical field, for example, as substitutes of articular cartilage or other bio-tissues.