Put the strawberries, rhubarb, sugar, and 3 cups of water into a deep saucepan on medium heat. Add more water if it doesn't seem liquid enough. Simmer for 20 minutes or so, until the rhubarb and strawberries are easily mashed with a potato masher or fork.

Pass the pulp through a fine-mesh sieve and work it with a spoon. This mashes the pulp very finely and makes sure you don't have any stringy pieces of rhubarb left over. After pushing the syrup through the sieve, pull out any stringy-looking pieces from the mash and add the rest of the fruit mash back into the separated syrup.

Juice the grapefruits, taking care to remove any seeds. Add the juice from the grapefruits, as well as the zest from 1/2 of one of the grapefruits into the pulp.

Using a mortar and pestle, mash the mint leaves together until they form a paste. If you don't have a mortar and pestle, you can use your knife to chop the mint very, very finely. Add the mint to the pulp and whisk it all together. Taste and add more sugar (1/4-1/2 cup) and water (1/2-1 cup) until it doesn't taste bitter and is slightly more liquid than applesauce. Chill the pulp in the fridge overnight.

When you're ready to assemble the drinks, use a ratio of 1 part pulp to 2 parts ice and 2 parts club soda. Ice first, then pulp, then club soda. For one drink, you'll use approximately 1 cup of ice, 1/2 cup pulp and 1 cup club soda. Enjoy!