How does Paan stain mouth?

Chewing paan without kattha will not stain the mouth. The mouth turns red on chewing paan with kattha because of a chemical reaction which releases a red coloured compound. Kattha contains a substance called catechu. Catechu reacts with oxygen in an alkaline medium provided by the lime applied to paan to give catechu-tannic acid. Catechu-tannic acid is the red compound which stains the mouth.