Clean crowd gate vector used for crowd control. This gate vector is very simple so you can use it in lots of projects. One gate vector in this download.
A stylized representation of the concept of chain reaction, more specifically at the atomic level (as in nuclear fission by neutrons) -- no intent of being scientifically correct, just an artistic interpretation. Drawn in Inkscape.
This is clipart converted and broken apart that has been released into the public domain because the document says so, and its from NASA, a USA government agency. The publication is called "The Brain in Space:"
http://www.nasa.gov/audience/foreducators/topnav/materials/listbytype/The.Brain.in.Space.html