How Would You Know?

I'm looking over some PDFs for a chemistry solutions book, and one of the solutions says:
When a particle is absolutely small it means that you cannot observe it without disturbing it.
When you observe the particle, it behaves differently than when you do not observe it.
How can they possibly know how it behaves when it's NOT observed, since knowing the answer to that would require that you observed something in the first place?! I know there is some scientific rationalization for things that are "not observed" but my Left Brain is going, "Wait... WHAT?!" and threatening to go all Blue Screen of Death* on me.
(Yes, you're right, my brain actually runs on Apple, but if it crashes I blame Microsloth like any other Mac Fanatic. :)
*Credit for BSOD graphic goes to Tully.