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You will never use half the supplies you were told you'd absolutely need.
Those triangle-y pink erasers you stick on the end of pencils? Six packs of Kleenex? Really? And yet on the first day of fourth grade, you bought every supply on the list believing that the teacher might call on you and suddenly demand you present all 17 of your 17 black (not gel!) pens. Only after many, many years did you figure out that the teacher does not do this—ever. Further, if you were going to need, say, a graphing calculator, your teacher usually brought it up the day before, allowing you to stop by Staples with Mom and buy one.

On your next trip to the computer store, as the salesman tries to convince you that you need office software (at 200 bucks a pop) plus an external hard drive and a few $50 cords, pause and think of your never-used-yet-always-purchased protractor. Then just buy the computer you came for and come back later for the extras you really need. People are always going to tell you that you need stuff; they are trying to help. But you know best what you will actually use—and what you won't.