As I've mentioned before, I really enjoy reading Letters of Note - and that's largely because editor Shaun Usher does such a fine job unearthing moving and interesting letters. In this 1999 note, Christopher Reeve of Superman fame offers some poignant advice to middle school students:
You are the future. You can make a difference. Perhaps you will work in a research laboratory that will help discover the treatments that will enable me to not only sign my name, but to get up and walk again.
I highly recommend reading his letter in its entirety.