67.) Your possessions will always expand to fit the space in which you live.

It's not particularly geeky, but I wish someone had told me this when I was young.  It's easy enough to move from a bedroom in your parent's house to a prison cell of a dorm room or from a dorm room to an apartment, and it's simplicity itself to move from an apartment to a house.  It's just near impossible to do it in reverse.  It's something to reflect on when the time comes to go off to college, especially if your post-graduation plans include living abroad.