The classic example is salad. What is a salad? A mixture of lettuce, plus dressing? What about tuna salad? Pasta salad? Maybe its a mixture of savory ingredients plus some acidic liquid dressing? This would make spaghetti and meatballs a salad, and what about fruit salad?
If you really chase your tail on defining what salads are you'll come to wonder if the word means anything at all. And yet we all seem to competent in employing it somehow. The way to explain this phenomenon is by appealing to the Use Theory of language, which states that the meaning of terms comes not from abstract definitions but from how the words are used. Words are no longer carriers of pre-assembled meaning packets, but they are tools we can use to execute 'locutionary acts' (see J.L. Austin). If you try to figure out what a word means by trying to consolidate all of its usages, you'll get nowhere. The word 'Knowledge' is impossible to define in general, but any given usage is easily grasped because in each case the word is being used to express something slightly different (See Wittgensteins 'Family Resemblance' theory). If you want to know what a word means in some utterance, don't ask for the definition, ask how it is being used. We all know what salad 'means', not because we know the definition, but because we know all of the ways it cam be used to express states of affairs, or do other locutionary acts.