the complete graph k5

this is the complete graph k5

http://knowledgerush.com/wiki_image/f/f0/GraphK5.png




how can we prove that k5 is the onlysimple nonplanar graph wuth nodes or fewer?

how it could be drawen with less that 5 nodes(Wondering)

hi there
if i delete one edge of this graph that will give a smaller graph which planar so i think we could not find a smaller graph and nonplanar

please help me if that correct idea