This is a short list, so it won't be hard to remember. You can have water of any kind, including sparkling and mineral, just avoid any that contain flavours or sweeteners. Some experts say you can have a slice of lemon in your water or a splash of apple cider vinegar. If this is necessary for your compliance, go for it, but if not, drink it plain.
Black coffee and herbal tea is also allowed, just avoid cream or sweeteners — this includes Bulletproof Coffee. Avoid BCAAs, smoothies, protein shakes, Gatorade, and broth. The point here is that if you take in calories, then you're not technically fasting.
Image Source: Getty / Onfokus