I was chatting with a teenage family friend and somehow the topic of retirement came up. He was puzzled--how could anyone ever retire and have enough money to spend? He's heard that "you have to save for retirement" but it seemed to him that even if you saved half of what you earn, you'd have to work for N years to have 2N years worth of expenses. So I explained about compound interest and showed him some graphs. We played around with various scenarios (historical stock market returns, low-interest savings account, different levels of post-retirement spending).
At the end he said, "Wow! They should teach this stuff in school!"