particle collision

        #write temp vars then test  wall

        #check for walls:
        if 0 > x or x > WIDTH:
            dx = dx * -1
            print 'x is out', x
            #if dx2 > 0:
                #dx2 += 1
            #elif 0 < dx2:
                #dx2 += -1

            #else:
                #print dx2, 'x probably equals 0'

        if 0 > y or y > HEIGHT:
            dy = dy * -1

            print 'y is out', y

            #if dx2 > 0:
                #dy2 += 1
            #elif 0 < dy2:
                #dy2 += -1

            #else:
                #print dy2, 'y probably equals 0'