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The ultimate transformers, these two-step treatments (also called modeling masks) start as a powder containing algin, a derivative of brown algae. Mix it with water or an accompanying gel and the formula congeals, morphing into a rubberlike layer. As the mask dries, it creates a seal, driving moisturizing molecules into dehydrated skin. After 15 or 20 minutes, you can revel in peeling it off, revealing softer and plumper skin.

Best for: Death Valley–level dryness.

Try: Dr. Dennis Gross Hyaluronic Marine Hydrating Modeling Mask ($46 for four treatments;