In a tall thin glass, crush part of the mint with a fork to coat the inside. Add the sugar and lemon juice and stir thoroughly. Top with ice. Add rum and mix. Top off with *chilled* club soda (or seltzer). Add a lemon slice and the remaining mint, and serve.
Serve Mojito in a Parfait Glass
1 1/2 oz Light Rum
3 tbsp (fresh) Lemon Juice
2 tsp Sugar
3 (Sprig) Mint