While many people do know this, I have found that many people do not and wanted to share the info.
Grapes and raisins, even just one, can be fatally toxic to your dog. There is something in the grapes/raisins that causes the kidneys to shut down and can lead to death. No one is quite sure yet what causes this (some think it may be something in the skin, but again, not sure), and it doesn’t happen to all dogs. There is no way to determine if your dog will be affected until it is showing symptoms. Once your dog is showing symptoms, there is already significant kidney damage and your pet will require hospitalization, and critical care, and there is no guarantee they will make it.
Please do not feed your dogs grapes or raisins. You just never know.