This happened to my brother but I'll say it anyways. He was in 4th grade and in the computer lab. You pick a random computer to go on. Somebody made the background image a black dick, but people didn't know what it was. My brother didn't know but had that image stuck in his mind so he was able to figure it out a couple years later. Teacher asked him to go to another computer while she removed it. Funny thing is he told me the kids around him, and himself, thought it was a thumb and compared it to the dick on screen.