"All of these people are your age. They always will be. Once you leave here, you'll remain in these books...forever 18...and others will look back on you even after you're long gone."
It was a sobering thought for many. The faces they were looking at seemed both ancient and young at the same time. The years had slipped away, forward and backward, in an instant. While young people aren't great a understanding longevity (the fact they're not going to live forever), I believe for a moment they saw a glimmer of how brief their moment in high school really is.
Of course, I'd be foolish to take credit for this idea...I stole it from Alan Keating (Robin Williams) in Dead Poets Society as a way to introduce the poem "To Make Much of Time". It's an easy concept to understand...but only to those for which it is already too late.