Frozen Watermelon Slushies/Mojitos

Spread your watermelon cubes on a sheet tray and freeze until frozen. Other varieties of melons will freeze harder, so you won't need to freeze them as long. Add the sugar to the lime juice and stir until the sugar dissolves. Place the frozen watermelon, sweetened lime juice, rum if using, and blend until thick but smooth. Add the mint and pulse until chopped but not too finely. Pour into two glasses and add a splash of seltzer if desired.