Pour the warm water into a cup and add the butterfly pea flowers, then stir and let them soak for a few minutes. The liquid should turn a deep sapphire blue; if it still looks pale, steep a bit longer or add more flowers.

Set a mesh strainer over another cup or bowl and strain out the flowers. You want a clear blue tea with no petals left behind.

Stir the granulated sugar into the warm tea until it fully dissolves. The mixture should look smooth, not cloudy with sugar grains settled at the bottom.

Add the lemon juice to the sweetened tea and mix well. The color will shift from blue toward purple almost immediately.

Fill the serving glasses with ice. A full glass of ice helps chill the drink fast and slows dilution a bit.

Pour the lemonade mixture over the ice until each glass is about halfway full. You should see a vivid jewel tone in the glass before the soda goes in.

Top each glass with soda water and serve right away. The drink should taste bright and fizzy, with the color looking especially striking in a clear glass.