Hafsah, the wife of the Prophet said that the Prophet would use his right hand for eating, drinking, and (putting on) his clothes, and would use his left hand for other matters. (Hasan)
(Sunan-Abu-Dawud volume 1, hadith number 32)
The Hadith shows that the invocation by the Messenger of Allah (pbuh) was answered. Using the left hand to eat or drink is a major sin.
The natural sense of purity and cleanness dictates one should use one’s right hand to eat or drink. Using the left hand for this purpose is abominable because the left hand is used to remove impurities after relieving oneself.
Children should therefore be taught Islamic manners regarding cleanness and purity from an early age.