instruction:
Two analogies that signify affordances are given in the form "A : B. C : ?". Affordance is the possibility of an action being done on an object, for example book is an affordance of writing. The phrase "A : B" implies that B is an affordance of A. Your task is to replace the question mark (?) with the appropriate affordance of the given action C, following the "A : B" relation. Your answer should be a single object without further explanation.
question:
sail : boat. pet : ?
answer:
cat


question:
attack : enemy. cure : ?
answer:
disease


question:
say : word. operate : ?
answer:

machine