In some sense you don't have to recognize the Pentagon to know it when you see it. Everyone knows that the words "the Pentagon" are used as a metonym for the US

In some sense you don't have to recognize the Pentagon to know it when you see it. Everyone knows that the words "the Pentagon" are used as a metonym for the US military, and it's pretty much the only pentagonal building. So if you saw a pentagonal building you'd say "Hey I bet that's the Pentagon" even if you'd never seen it before.