You're thinking of the Heisenberg Uncertainty Principle, which states that the act of observing something changes it, and that you can only ever know with certainty the direction or position of an atom.

A lot of people confuse this with Shroedinger's Cat, which is a related but different ball of wax.

Niels Bohr put it best when he said "anyone who thinks they understand quantum physics is wrong."

 

Mike Oxbigger -

Here you go.