in many cases practical teaching is more effective than theoretical teaching. this is what the prophet (peace and blessings of allaah be upon him) did. jubayr ibn nufayr reported from his father that he came to the messenger of allaah (peace and blessings of allaah be upon him), who called for water, then said,

“do wudoo’, o abu jubayr.” abu jubayr started with his mouth, and the messenger of allaah (peace and blessings of allaah be upon him) said, “do not start with your mouth, o abu jubayr, for the kaafir starts with his mouth.” then the messenger of allaah (peace and blessings of allaah be upon him) called for water, and washed his hands until they were clean, then he rinsed his mouth and nose three times, washed his face three times, washed his right arm up to the elbow three times, and his left arm three times, wiped his head and washed his feet. (reported by al-bayhaqi in al-sunan, 1/46; al-silsilat al-saheehah, no. 2820).

we may note here that the prophet (peace and blessings of allaah be upon him) deliberately put this sahaabi off from doing an incorrect action by telling him that the kaafir starts with his mouth; the meaning may be that the kaafir does not wash his hands before putting them in the vessel (i was told this by shaykh ‘abd al-‘azeez ibn baaz when i asked him about the interpretation of this hadeeth), and that this is not hygienic. and allaah knows best.