A wrinkle is a fold, ridge or crease in the skin, similar to those that can form on the surface of fabric. The medical term is rhytide.
Skin wrinkles are common as a result of aging , reoccurring sleeping positions, loss of weight, or as the result of being under water for long periods of time. Age wrinkling in the skin is affected by constant facial expressions, aging, sun damage, smoking, poor hydration, and various other factors.
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Wrinkle (skin) Facts for Kids. Kiddle Encyclopedia.