Blue pea lemonade, and why it changes colour

This is the recipe we hand to anyone who asks what butterfly pea actually does. It takes ten minutes, and the last step happens in front of your guests.

For two glasses

• 2 tsp dried blue pea flowers
• 250 ml just-boiled water
• Juice of 2 limes (or 1½ lemons)
• 2 tbsp honey or sugar, to taste
• 500 ml cold water, plenty of ice

The method

Steep the flowers in the hot water for a full seven minutes — you want a serious, inky indigo — then strain and let it cool. Stir the citrus juice and honey into the cold water and divide between two tall glasses over ice. Now pour the blue tea slowly over the top, and watch: where it meets the citrus it turns from indigo to bright violet, settling into layers you can stir together at the table.

The chemistry

The blue comes from anthocyanin pigments called ternatins, and anthocyanins are natural pH indicators: neutral water holds them blue, acid flips them purple-pink. The lime isn't a garnish — it's the reagent. The same trick works in tonic water, in gin, and over a lemony iced tea; anything acidic will throw the switch.

No caffeine, so the colour show works as well at nine in the evening as at five.