Place 6–10 fresh mint leaves in a sturdy glass and add the simple syrup (or agave, if you prefer). Gently muddle them together—just enough to release the mint’s oils without tearing the leaves apart. You can use a muddler or the back of a spoon.
Pour in the fresh lime juice, then fill the glass with crushed ice.
Give everything a gentle stir to combine.
Top it off with club soda or sparkling water.
Garnish with a sprig of mint and a lime wedge or wheel, and enjoy right away.