There are plenty of cocktails for you and your mates to try when visiting the 492 bar on King Street in Charleston, S.C , but few will impress as much as the Disco Sour,  a cocktail that changes color from pale yellow to deep violet while you drink it.

The magical mood-ring ingredient in the Disco Sour is the butterfly pea flower. The blossom, which goes by various names, is native to tropical Asia and is a striking blue. That vibrant hue has made it popular as a natural food and drink colouring for centuries.

The flower slowly transforms a drink’s colour to purple when an acid like lemon juice is added. In recent months, that special effect has made it a hot ingredient in cocktail bars around the country.