Everyone’s buzzing about “serum oils,” but where the heck do they go in your routine?

In recent years, the Internet has run wild with things that aren’t quite as they seem. Is the dress black and blue, or white and gold? Is the shoe pink and white, or teal and grey? Is she saying “Yanny” or “Laurel”?! The trend has recently taken hold in the beauty world, too, with the serum/oil hybrid—a new product category that has us asking, “Is it a serum, or is it an oil?” Well, as with all of the other examples, this isn’t an “either/or” situation—it’s both. Which means figuring out where the heck to use it can be about as challenging as trying to convince your coworkers that the dress is, in fact, black and blue (and I will fight you on that).

To understand what serums and oils do when they’re combined, it’s first important to understand how they function on their own. Serums, the central star of any skin-care routine, are known for having high concentrations of actives like vitamins, peptides, or AHAs/BHAs. They’re generally lightweight, which means they can penetrate the skin more deeply and deliver all of those important ingredients deep within its layers. Oils, on the other hand, tend to be made with essential oils and botanical ingredients, and are meant for locking in moisture. “Oils have larger molecules, so they can only penetrate the outer layers of the skin, which is what makes them different from serums,” says Cindy Kim, co-founder of Silver Mirror Facial Bar.

A “serum oil,” then, is basically what you would get if these two things had a baby. The result? A two-for-one offering of active ingredients plus hydration, which does the work of a serum with the consistency of an oil. “These products are especially beneficial during the colder, drier months because they penetrate nutrients into the skin, deliver vital fatty acids to the outer layers of the skin, and then seal in the moisture,” says Kim, adding that they help strengthen and protect the skin barrier, too, which gets compromised in cold weather.

To get the most bang for your serum-oil buck, apply it to clean skin before your moisturizer so that it can penetrate the most deeply. With some serum oils, you may not even need a moisturizer on top since they’re hydrating all on their own, but if you’ve got dry skin, it’s probably for the best to lock in all that active ingredient goodness. Here, some serum-oil hybrids that we love for getting the (double-duty) job done.

Odièle Rose Serum, $125

If I were stranded in the Arctic, this is the one skin-care product I’d bring with me. It’s made with a mix of antioxidants and oils that soothes and hydrates skin so beautifully that it’s often the only thing I use in my routine in the winter. Plus, it smells so good that people stop me to tell me how good my face smells.

Glossier Future Dew, $24

Glossier’s “oil serum hybrid” is the latest to hit the market, and is meant to give your skin that “fresh from a facial” look all day long. Unlike most oil serums, this one should be applied on top of your moisturizer for long-lasting dewiness.

Immortelle Reset Overnight Reset Oil-In Serum, $59

Go to bed with your face slathered in this serum, and wake up with smoother skin. It’s made with a mix of oil extracts known for their line-diminishing properties, and feels like pure luxury to apply.

Speaking of double duty products, these are some of our favorite multitaskers in the makeup category. But one beauty hybrid you may want to keep out of your routine? 2-in-1 shampoo/conditioners.