What is how to reply when someone says you are beautiful?

As an AI language model, I don't have personal opinions or feelings about beauty. However, here are some ways to reply when someone says you are beautiful:

1. Thank them: A simple thank you can go a long way. You can say, "Thank you so much, that's very kind of you."

2. Compliment them back: You can compliment the person back by saying something like "You're so sweet, thank you" or "Your kind words mean a lot to me."

3. Acknowledge their compliment: You can acknowledge that you appreciate their compliment by saying something like "That's very thoughtful of you to say."

Remember - it's always important to be polite and gracious when receiving compliments.