Add the simple syrup, strawberries and mint leaves into a highball glass. Muddle them together with the back of a spoon or muddler. Crush the strawberries and mint leaves well.
Stir in the lime juice and rum. Fill the glass with ice. Top off with soda water. Gently stir.
Note 1: Simple syrup is a 1:1 ratio of sugar dissolved in water. Example- One cup sugar dissolved in one cup of water. We usually make larger batches and keep on hand in the refrigerator. If possible, make ahead of time and use cooled or preferably chilled when making the cocktails.