The Three Kingdoms

Today X, who has been preparing for her PhD qualifying exams, asked me sheepishly:"What are the three Kingdoms of, you know, biology?"
 
I was puzzled:"Three kingdoms? I thought there are more than three! See, Animalia, Plantae, and..." I found I didn't know the correct terms for other kingdoms, so I go, "the one with bacteria, you know, the prokaryotes, and at least there is one with the fungi..."
 
I swear I once knew this. I had learned this repeatedly in high school, in college and when I was a microbiology TA a couple of years ago. At least three times, and still I couldn't remember.
 
I guess I am not Ken Jennings, after all.
 
Nevertheless, I felt ashamed of my ignorance. So I pulled out a biology text book from the shelf, blowed off the dusk, and learned some solid biology knowledge.
 
"The one with bacteria" is actually 2 kingdoms:"Eubacteria" and "Archaebacteria". "The one with the fungi" is actually called "Fungi". Besides these, there is another kingdom called"Protista", a kind of junk-basket category for organisms that don't fit into the other eukaryotic kingdoms.
 
So why did X ask for 3 kingdoms? There was no answer in the book so I did some research online and found out that what she meant was "domains". There are three domains: Bacteria, Archaea, and Eukarya.
 
Eureka!
 
While X was writing these down, she sighed:"If the boss knows I am studying this kind of stuff..."
 
Yeah. It's pathetic, isn't it? So basic, yet so elusive. The more you learn, the less you know. What's the point of learning then?
 
The take-home lesson: the storage capacity of human brain is so inadequate.
 
Or we are just getting old.