Semantics

I find it strange that we call them "car keys." Only one key is required to unlock one's car. Yet, we always say, "I can't find my car keys," not: "I can't find my car key," or: "I can't find the key to my car."

And while we're at it, why is it a "pair" of pants? Pair of legs, yes. But since we also refer to the item of lower body attire with a singular-word term, just "pants," then why is the "pair" piece necessary?

And peanuts. When someone refers to a "peanut," you can picture one of three things, am I right? Yes I am.

1, you can picture an entire peanut, shell intact:



2, you can also picture one of the two individual nuts inside the shell:



3, you can break one of those individual nuts in half along its natural seam, and still we call each of those pieces a "peanut" as well (item in foreground of picture below, which I have dubbed "Evolution of the shelled peanut"):



So riddle me this. Which piece exactly is the peanut??? Please don't tell me life is simple when there are such questions as this in our world.