You know, those things you have in your bag or your pockets that you absolutely can't leave home without and would rather be late for wherever you're going than not have them.
I used to be like that with my iPod / discman when I was at school, I'd be an hour late a couple of times every week because I couldn't go unless I had it, and I only lived about 8 minutes away.....
To stop that happening, I keep the most of it in a mini-bag that stays inside my actual bag:
Chalk (a couple of different colours)
Paper (2 or 3 sheets)
Pen (black biro)
String or Thread
Aside from those, I need:
Bottle of Water
Book and/or Nintendo DS
Co-Codamol and Ibuprofen