Why Do You Know That?

It worked a little while back, so let's try "Why Do You Know That?" again. It's the converse of "auto mechanics to quantum mechanics," where the idea now is to contribute those bits of knowledge that seem really cool even if they are not directly applicable to anything.

My contribution:

"I" and "MIX" are the only numbers expressible in Roman numerals that are also English words.

Next to Stonehenge there actually was a Woodhenge, although contrary to Eddie Izzard's routine, there is no reason to believe there was also a Strawhenge that was blown down by a big bad wolf.

Isaac Newton's father died before he was born.

Baltimore and Saint Louis are the only cities that in the country that are part of a state, but not part of a county.

So, what do you know and why do you know it?