Simmer the strawberries (or fruit of your choice) and sugar in water for 20-30 minutes.
Cool to room temperature. Strain out the strawberries.
Transfer the strawberry juice to a gallon glass jar. Add the whey. Cover with a piece of fabric, coffee filter or muslin (rubber band appropriately).
Allow the mixture to sit for 3 days (72 hours). The longer you allow it to go, the less sweet it will be as the bacteria will feed on the sugars.
Transfer the soda to your swing top bottles, make sure you allow a few inches of space at the top.
Keep the soda bottles at room temperature for 1-2 additional days. Check the bottles each day -- open up to allow some of the pressure to escape (do this over the sink!)
Move the bottles to your refrigerator so slow down fermentation. Serve chilled over ice.