One kind of answer certainly would consist in describing a series of intermediate cases. One might say that the case which looking in your memory for something is most similar to is not that of looking for my friend in the park, but, say, that of looking up the spelling of a word in the dictionary. And one might go on interpolating cases. Another way of pointing out the similarity would be to say, e.g., “In both these cases at first we can't write down the word and then we can”. This is what we call pointing out a common feature.