```function enemy.update(enemy)
  enemy.x = enemy.x - enemy.spd
  if enemy.x < 0 then
    enemy.x = 900
    enemy.y = math.random(25,575)
    enemy.spd = math.random(1,10)
  end
end
-- below is the same as enemy.update(enemy) seen above
-- the : operator when defining a function automatically makes the first argument's name 'self'
-- self's value is the first argument passed in
 -- myenemy.update(myenemy) passes in myenemy as the first argument
 -- myenemy:update() does the same thing
function enemy:update()
  self.x = self.x - self.spd
  if self.x < 0 then
    self.x = 900
    self.y = math.random(25,575)
    self.spd = math.random(1,10)
  end
end
-- this is NOT the same as either of the two above
-- the first argument is still called self, but you never use it
-- instead, you used enemy
-- the reason this is different, is because arguments are automatically local variables to the function

function enemy:update()
  enemy.x = enemy.x - enemy.spd
  if enemy.x < 0 then
    enemy.x = 900
    enemy.y = math.random(25,575)
    enemy.spd = math.random(1,10)
  end
end
```