Nobody in my Critical Thinking AS level class knew what an armadillo was. Had they never seen the Dime ads?
On a university field trip (university), we were mapping the rock formations in some Spanish hills. We were put in pairs or small groups, and then dumped in the middle of nowhere with maps, compasses and a packed lunch, and were picked up again in the evening. (It really was in the middle of fuck-all, so if you needed a wee all you had to do was squat in the nearest obliging bush. I lost my outdoor-pooing virginity that trip.) As we explored the area we'd encounter other groups/pairs on the way, and we might compare info. My group met a pair of girls who were having trouble taking their readings. One of the things we had to do was dip and strike; I can't be arsed to explain all of it, but it does involve knowing which way is north. One of the girls was waving her compass around and saying 'but which way is north?' We told her to hold it flat and look which way the needle pointed. That just confused her even further: 'but when I turn around the needle moves!' Christ.
Summary: University student doesn't know how to use a compass.

Asked by someone in my Philosophy A level class: "Why doesn't all the water on the bottom of the Earth fall off?"
When I told the class the Reese Witherspoon joke ("Did you hear the news about that Hollywood actress who got stabbed? That blonde woman, whatsername... Reese something." "Witherspoon?" "No, with a knife!"), everybody laughed except her, who didn't get it and thought we were all being meanies for laughing at someone getting stabbed.

I did 9 months' work in a charity shop, so not exactly a strict interview process. One of the things we had to do at the end of the day was cashing up (exactly the same as paying in a personal cash deposti: count all your/the till's money and fill in a banking slip), which usually fell to whoever had been working the till. When we got a new person, a woman 10 years my senior who was studying business at college, the first time she did the banking she was concerned because we were 45p short: when she added up all the money with a calculator, it fell short of however much the till said we had. I had a look, and discovered that she was entering the sole 5p piece into the calculator as 0.5. After five minutes I had failed to persuade her that 0.5 is the same as 0.50, and that she needed to punch in 0.05. In the end she just took my word for it.
Summary: Grown woman can't grasp decimals.

I used to work at another job with a very thick girl. She had never heard of the Beatles. One day I came in wearing a Yellow Submarine tee, and she read it and said, "the beatless? What's that?"

A zoo-keeper giving us a talk about her fruit bats told us they were mammals. She asked the audience if anyone had thought they were birds, and three adults put their hands up.

My grandfather wrote 2 books but had never heard of algae.

I'll add more as I think of them. Edit: I don't want this to be obnoxiously long, so I'll put more in the replies.
( Bob Todd whee, Thu 18 Mar 2010, 17:53, 3 replies)

I know someone who didn't know celery came in bunches, as he'd never bought any or looked at it in the shop.
I said something about '2 bunches of celery' and he said 'how big is a bunch?' I was floored.

Someone who claimed to have been using the internet for 15 years didn't know how to ping a site, and didn't even know what that meant.

My dad works with someone who doesn't know the difference between a desktop and a laptop.

My mum works for an IT company, and used to have a colleague who was grandfathered in from when the company just did editing. She panicked because there was 'a Chinese character' in the middle of some text on her screen and she didn't know what to do. It was her mouse cursor over a letter.
( Bob Todd whee, Thu 18 Mar 2010, 22:58, closed)

if I wanted to know
what "ping" is, I would google it, does that make me thick?
(whiskas loved by 8 out of 10 cats, Fri 19 Mar 2010, 18:23, closed)

The fact that you would google it
makes you less thick than anyone who asks what something is WHILE THEY'RE ON THE INTERNET AND CAN FUCKING LOOK IT UP.
( Bob Todd whee, Fri 19 Mar 2010, 23:45, closed)