Rinse limes hot, rub dry, cut into small pieces, squeeze juice. Wash mint, shake dry and pluck leaves from the stalks, put some leaves aside for garnishing. Wash the rhubarb, cut in half and cut according to the height of the glass.
Put lime juice, elderflower syrup and rhubarb syrup into a container and mix well.
Press mint leaves and fill the glasses with ice, spread rum and syrup on the glasses, fill up with mineral water. Garnish with mint and rhubarb and leave out more mineral water for children.
For rhubarb syrup: Clean, wash and cut 2 kg rhubarb into pieces. Boil 450 g sugar and 2 l water in a pot while stirring. Add rhubarb, simmer for about 30 minutes. Pour the rhubarb through a sieve and collect the stock in a second pot. Squeeze the rhubarb residues in the sieve well with a ladle. Reduce the stock to approx. 3/4 l - this takes 30-40 minutes depending on the size of the pot. Add the juice of one lemon to the rhubarb stock after 20 minutes and bring to the boil. Let the syrup cool down briefly. Pour it still hot into a prepared clean bottle, close it tightly and let it cool down. Shelf life: 10-12 months