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Close-up of a gloved hand applying a dental filling to a molar with a tool, under blue UV light.

Close-up of a gloved hand applying a dental filling to a molar with a tool, under blue UV light.

Dentists use a soft, clay-like material to make a filling for a cavity. The filling stays soft under the light of the dentist’s lamp, but it hardens when the dentist shines an ultraviolet flashlight on it. See more