How Do We Know Any of This?    
Deflected Probe
From http://particleadventure.org

Try it yourself! In the following pictures, there is a target hidden by a black cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?



The First Target Was:
    A Wedge


The Second Target Was:
    A Circle