The biggest difference between a serum or elixir and a cream or lotion is what the formulation DOESN'T include.  Always check the full ingredients list.  Serums and elixirs are thinner, containing fewer lubricating and thickening/filler agents such as dimethicone, carbomer, wax, or xanthan gum.  Because active ingredients are more expensive than thickeners, serums and elixirs are the costliest product in most skin care lines.  They are made up of very small molecules so the skin can absorb them.  As you can see in this example, xanthan gum has a very high molecular weight, which is OK in heavier products (lotions & creams) to lock moisture in and form a protective barrier over the skin.  However, thickeners have absolutely no business in your high-end, concentrated serums because thickeners can also lock active ingredients out, which are what you're supposed to be paying for.  That's the reason moisturizer lotion or creams tend to cost less and are always the final step in a skin care routine. Shop wisely and try Beleza by Z Hyaluronic Acid Hydrating Elixir for dewy, hydrated skin!