Hyaluronic acid

Morning and night if you like — there is no adjustment period and no cumulative irritation. The only rule that matters is moisture: apply it to still-damp skin and seal it with a moisturizer. On dry skin in dry air it can do the opposite and leave you feeling tighter.

Rarely. It is a cheap, stable ingredient with no meaningful patents: the gap between an €8 serum and a €60 one is seldom the hyaluronic acid itself but the rest of the formula, the texture and the brand. If hydration is what you are after, look at the price per millilitre before the molecular-weight claim.

It's a humectant: a molecule that attracts and holds water (up to about 1,000 times its weight). On the face it gives immediate hydration and a temporary plumping effect that softens fine lines caused by dehydration.

No. As a topical it hydrates the surface layers and temporarily plumps the skin, making wrinkles less visible while you use it — but it doesn't remodel skin the way retinoids do. The true "filler" effect only comes from injectables.

They do different things. High molecular weight sits on the surface forming a hydrating, soothing film; low molecular weight penetrates a bit further and hydrates deeper into the epidermis. The best serums (including many K-beauty ones) combine several weights.

Yes. It's lightweight, non-comedogenic and hydrates without adding oil — exactly what dehydrated oily skin (which overproduces sebum to compensate) needs.