S1: The first great discovery that man probably made was that of fire. P: Fires sometimes occur by themselves in the forests perhaps by rubbing together of flints or something else. Q: We light a fire now by a match. R: In olden times fires were made by striking two flints against each other till a spark came for setting fire to a dry thing. S: But matches are quite recent things. S6: The animals were not clever enough to learn anything from this. Find the correct sequence of P, Q, R and S.