1-1/2CupsClub soda(depending how big your glasses are)
Divide the lime chunks and mint leaves between two highball glasses. Use a muddler to muddle until the limes and mint break down and are juicy.
Divide the syrup and watermelon between both cups and muddle again.
Divide the rum between both cups, followed by the club soda and a few ice cubes.
Stir and SIP!
*To make simple syrup just combine 1/2 cup sugar and 1/2 cup water in a small saucepan over medium heat. Once the sugar is dissolved remove from the heat and cool. Store in an airtight container in the fridge.