You can give type O blood to blood groups A and AB. You can give type A blood to type AB. You cannot give type AB to anybody except AB.

Butter knives in our kitchen work the same way. You can use a knife that has previously been used for Adrian’s dairy-free margarine on both butter and Ingrid’s liver pâté. A knife that has been in contact with butter can be used for liver pâté but not for margarine. A knife with liver pâté on it can not be used for anything else.

The complication is that remains of butter and margarine are almost impossible to tell apart. So whenever Adrian wants a sandwich, we have to get a new knife for him. This is why there is an almost-constant shortage of butter knives in our kitchen even though we have half a dozen.

(On weekend mornings there is also orange marmalade to be taken into account but that luckily does not look like anything else and can be eaten by everybody.)