3 tablespoons rice vinegar (not seasoned)
2 tablespoons sugar
1 (1-inch) piece peeled ginger, cut into thin matchsticks (1 tablespoon)
Heat vinegar with sugar in a small saucepan over medium-low heat, stirring, until sugar has dissolved. Remove from heat and add radishes, then stir in ginger. Transfer to a small bowl and marinate, chilled, at least 2 hours.