A genuine K-Beauty phenomenon, sheet masks are all kinds of awesome. Not only do they come drenched in powerful hydrators, brighteners, and soothers, they can also bolster any serum you smooth on underneath by keeping it from evaporating. But as you’ve probably heard, not all sheets are created equal.Those made of plant-based bio cellulose, fermented coconut juice, and hydrogel are the most occlusive, notes Wilson. Meaning they cling like crazy, forming a tight seal, and thus do the best job of driving in both the active ingredients imbued within the sheets themselves and whatever you combine with them.Capitalizing on this power, Alicia Yoon, founder of Asian importers Peach & Lily , advocates what she calls sheet mask sandwiching: “Peel off the sheet about halfway through your treatment, when skin is nicely hydrated, and apply whatever serum you need that day; then place the mask back on top to further boost absorption,” she says, adding that Korean women have all kinds of creative ways of milking their mask materials.It’s worth noting, however, that some dermatologists are wary of the practice, since intensifying certain actives — think: acids, retinol, benzoyl peroxide, hydroquinone — can lead to irritation, as can unwittingly creating scary-strong combos when mixing and matching products. If you’re not totally sure of how the ingredients in your sheet mask will interact with those in your serum, play it safe by either a) choosing a mask that comes with an essence made to be applied underneath, like Sally’s Box Secret Garden Camellia Oil Ampoule Mask , or b) smothering a non-irksome serum (one with peptides or hyaluronic acid, say) with a hydrogel mask:“Hydrogels make great toppers, because they’re primarily for hydration and don’t typically contain the same anti-aging ingredients as bio cellulose or coconut sheets,” says Wilson. (Check out CremorLab Hydro Plus Aqua Tank Water Full Mask and Manefit Bling Bling Hydro Gel Mask .)