As teachers, we expect to build background knowledge. After all, students come to our classrooms with vastly different experiences, backgrounds, and educations. Still, sometimes they manage to catch us off guard with things they don’t know, as evidenced by the responses to this trending Reddit thread.

Redditor TapiocaPudding98 kicked things off by sharing that her ninth grade students “don’t know what country they lived in, didn’t know their birthday, and kids who have never heard of 9/11.”

What was your HOW COULD YOU NOT KNOW THIS moment? from Teachers

Those responses might seem pretty surprising—if you don’t teach high school. But other teachers were quick to add the surprising things their students don’t know. Some of these gaps are generational, others stem from background and education, and others…well, we can’t explain:

Some students can’t recognize basic shapes

Others don’t know their own addresses

A few don’t know their own middle names

Some AP students don’t know there are AP tests

Some high schoolers are surprised by the fact that Alaska is not southwest of California (because that’s how it is on the map)

Math teachers say they’ve had students who don’t know their odd and even numbers

Or their multiplication facts

Some students don’t know the months of the year

Or how to write an address and where to put a stamp

Or which letters are vowels

Or how to tell time

And finally…what’s in a deck of cards

We’re curious, if you teach high school—do these stories resonate? What surprising things have you found that students don’t know? Please share in the comments.

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High School Teachers Are Sharing the Surprising Things Their Students Don't Know