Then to bring the lesson home, I say, "Alright, flip the paper over and put two points on the page. Now we're gonna see who can get the most lines through both points."
Ready, set, GO!
They get the first line fast. Then they panic as they move the ruler and pencil searching for that elusive second line. Most of 'em end up looking something like this:
I tease them a bit and we all get a good chuck out of it. I know, I know. It's not nice to take advantage of these trusting impressionable children. But I don't care who you are, that thar's funny!
And, they never forget it.