Back in the mid 80s, we started to get our first personal computers. Prior to then, everything was on the mainframe computer.
You may remember in the early days of PCs there were lots of add-ons, one of these was an 'anti glare' screen. Well my co-worked wanted one of these, it was the means to a great prank.
I had figured how to print white characters on a totally black background, so I printed...........
If you are old enough, you recognize this prompt after the computer booted to DOS. I then placed the all black and white text printout BEHIND the anti-glare screen.
To make a long story short, it was close to an hour before the victim realized that his computer was actually on and responding, all he ever saw on the screen was C:>
We all had a great time playing along trying figure out what was wrong. We were tripped up when the co-worker turned the entire PC off and the image was still on the screen.