The chewing surfaces of back teeth are not flat. They are made up of ridges, valleys, and fissures that deepen toward the center of the tooth, creating a terrain no toothbrush can fully navigate. Bacteria and food debris settle into those grooves continuously, protected from bristles by the simple fact that the groove is narrower than anything designed to clean it. Decay starts there, works its way inward, and often reaches a meaningful depth before it shows up on an X-ray or causes any sensation at all.
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This is not a hygiene failure. It is a design reality. Molars evolved for grinding, not for resisting the modern diet of refined carbohydrates and sugary beverages that feed the bacteria living in those channels. Understanding this makes the logic behind sealants immediately clear.