The laws of the Paroh Aduma, the red heifer, are included in the category of the statutes or Chukim. The Midrash relates that a Roman aristocrat asked Rabbi Yochanan ben Zakkai to explain the strange laws involving the Paroh Aduma to him. Rabbi Yochanan replied “Just as a person afflicted with a certain disease is cured of illness by taking a certain medicine so too do the ashes of the Paroh Aduma when prepared in the prescribed way and dissolved water disperse the unclean spirit.” The Roman left satisfied with the answer he got. Rabbi Yochanan's students said that answer satisfied him but what answer do you have for us? To answer the students Rabbi Yochanan said, “The dead man does not make a person impure nor do the ashes make him pure again. My explanation therefore is that the law concerning the Paroh Aduma is that it is a Chok or a heavenly decree and we have no right to question it.”
The Sefer Lilmod Ulalmade says that in this response lies the answer to the reason of Chukim. We cannot always comprehend the things that Hashem gives man since our understanding is limited. It is sufficient for us to know that by obeying the Chukim we are doing Hashem's will.