In a small sauce pot, over medium heat, combine raspberries and sugar. Mix well.
Zest key limes and add zest to the pot.
Add juice of key limes as well and mix well. Cover with a lid but don't walk away.
After a couple of minutes on heat, start mashing the berries with a wooden spoon. Mash all the berries well.
Add water and mix well. Cover with a lid, lower the heat to medium-low and keep berry mixture simmering for about 5-7 minutes, stirring often. (Don't walk away because berries tend to raise and simmer over when cooking. If it happens, just raise the lid a little to let the steam out.)
Strain the mixture through a fine strainer to remove the seeds and solids.
Refrigerate the syrup until ready to use. Use cooled syrup in drinks.
Fill the glasses with club soda, about 3/4 way up. Add about 2 tbs of raspberry syrup per glass (more if you like it sweeter.)
(This recipe makes about a cup and a half of syrup, which is good for 9-12 8-oz drinks, depending on how sweet you like them.)Feel free to double the recipe for parties.