I remember playing a prank on my dad when I was a little kid. The Commodore 64s had an overheating issue where sometimes random exclamation marks would start to appear on the screen. Using the RND, PEEK, and POKE commands, I figured out how to clear the screen and start putting those '!' symbols all over the place. My dad initially got mad, thinking he would have to return yet another Commodore 64 to Toys 'R Us or wherever he got it, until I showed him that it was just some silly program that I wrote.