def step(self):
		"""Randomly move up, down, left, right or stay in one place."""
		
		choice = random()
		if choice < 0.4:
			self.x += 1
		elif choice < 0.6:
			self.x -= 1
		elif choice < 0.8:
			self.y += 1
		else:
			self.y -= 1

		self.x = p.constrain(self.x, 0, width-1);
		self.y = p.constrain(self.y, 0, height-1);
	def attract(self, m):
		force = p.PVector.sub(self.location, m.location)
		d = force.mag()
		d = p.constrain(d,5.,25.)
		force.normalize()
		strength = G*(self.mass*m.mass)/(d**2)
		force.mult(strength)
		return force