It's the understanding that things don't disappear just because you can't see them. I took a child development course where I learned that babies typically learn this concept by 8-9 months of age. Mothers might recognize that as the time when peek-a-boo becomes fun.
Well, apparently the concept of object permanence isn't just unknown to newborns, some college students don't get it either.
I am auditing a course on the University where I work. It's a grad level course and to be honest, quite boring. It is also held Wednesday/Friday afternoon from 3-5 which adds to the difficulty one has staying awake. But I, along with the other students in the class, manage to make it through each week at least looking like we are paying attention. However, there are a few students from Galveston, taking it by videocast, who don't even try. There is a camera on them and two projectors (one behind the professor that the students can see, and another behind us that the professor can see) that give us a nice close shot of them. They are aware of it's presence and will look at the camera when asking a question, but most of the time they put the microphone on mute and forget about it. I have entertained myself for quite a while just watching them talk, goof off, give each other high fives (for completing a crossword puzzle perhaps?), etc, all the while waiting for their shenanigans to bite them in the butt. And Friday they did.
We had a guest speaker who didn't quite know how the system worked but was doing his best. One of the students took the mic off of mute to ask a question which caused a shhh should to emanate from the speakers. This is normally the cue for the professor to pause and/or ask if anyone had a question but the guest didn't know that so he kept talking for about a minute. Just when someone on our side was about to tell him that there was a question, we hear the other student in Galveston, quite loudly, say, "WHAT is he TALK-ing about." The first student then quickly asks his question and mutes the mic again but it is too late. We are all now staring at the loud mouth student, just in time to see her mouth "Oh ****" and turn red. It was quite entertaining. Tomorrow is our first class since then and I am wondering if she will be up for a game of peek-a-boo yet.