This is about the coolest thing ever. It's a device that screws on to any two-liter bottle and allows you to carbonate whatever's inside that bottle. I have this system at home and use it primarily for soda water (I'm a nut for bubbles) and sometimes sparkling lemonade, but you can also use this to carbonate cocktails. It's especially handy for making bigger batches of bottled cocktails, and sometimes in the summer I'll make a case of Americanos for backyard parties. Anyway, it's part of the system I'm describing, so you need to pick one.