Picture a movie trailer that shows only the first scene of the film. The camera pans across a large number of clocks, a teenager enters and causes a giant speaker to explode with a pluck of his guitar. End trailer. Are you convinced yet? Excited to see Back to the Future?
Sometimes game demos don't even include proper levels, instead containing only the tutorial. "Welcome to an RTS. You select the troops by lassoing them with the left mouse button. Congratulations! You are the best I've ever seen at left-mouse-button lassoing. Please pay £35 to continue playing."