Matthew 22:42-45

Mace(i) 42 what think ye of Christ? whose son is he to be? they answered, David's. 43 how comes it then, said he, that David by inspiration calls him Lord, in these words, 44 " the Lord said unto my Lord, sit thou on my right hand, till I make thine enemies thy footstool?" 45 why then does David call him Lord, if He be David's son?