Use a zester or fine grater to carefully peel the thin outer layer of lemon and lime zest, avoiding the bitter pith underneath.
Add the chilled Sprite to your blender jar, then sprinkle in the freshly grated lemon and lime zests.
Blend on high speed until the mixture is snow-like and the zests are evenly suspended, about 20-30 seconds.
Check the consistency — it should be smooth, icy, and bubbly all over. If it’s too thick, add a splash more soda and pulse briefly again.
Pour the bright, bubbly slushie into a tall glass, allowing some of the icy bits to cascade over the rim.
Serve immediately with a straw or a stir stick to enjoy the fizzy, citrusy burst of flavor.