Not every early cavity needs a filling. Silver nitrate is a way to slow the progression of a small, surface-level cavity.
Silver nitrate is a clear liquid that contains silver ions, which kill the bacteria that cause tooth decay. Applied to an early cavity with a microbrush, it penetrates the porous, demineralized enamel or dentin and denatures the bacterial proteins, which stops the infection from progressing. Because it needs almost no anesthetic and goes on with a brush, the whole process is quick, quiet, and usually completely painless.
One honest caveat: on its own, silver nitrate slows decay but doesn't rebuild tooth structure. So it works best alongside good habits at home, regular flossing, a nano-hydroxyapatite toothpaste, and adequate vitamin D levels, which together help your teeth remineralize.