Why the Roman Empire fell...

...because it was too ungodly cumbersome to do proper math using Roman numerals, of course.
S. has been fascinated with Roman numerals since we've been reading Charlotte's Web. The chapter headings are all preceded by Roman numerals. So he's been pop-quizzing us and stumping me on several occasions.
"What's 100?"
"C"
"How about 99?"
"Good question..." -- I can think of about three different ways to put it together, but none of them sounds quite right. I go look it up.
"It's XCIX." -- Of course, it seems so obvious, looking at it.

Then, he wants to know everyone's ages in Roman numerals
"What am I?"
"V."
"How about Claire?"
"XI"
"And what are you?"
"XXXVIII" (ha- at least I'm still in the X's)
"What's Dad?"
"Dad is XL."
This cracks both of us up.