I heard this story from my teacher, the mashpia (chassidic mentor) Rabbi Mendel Futerfas of blessed memory.
When Rabbi Mendel was five years old learning Torah in a cheder in Russia, it happened that one of the boys forgot to bring his ink bottle and asked the boy at his side for some of his. "No," replied the latter. "I haven't enough; you should have brought from home." So the first boy had to ask someone else.
The teacher noticed this and said nothing, but a half hour later he asked the second boy if he could show the class an Aleph, a Bet and a Gimmel (the first three letters of the Hebrew alphabet). "Of course," answered the child as he pointed in one of his books. "This is an Aleph, this a Bet, and this a Gimmel."
"No," said the teacher. "You are wrong."
The boy was confused. "But teacher" he said, "this is what you taught us... this is what we have been reading for the last two years!"
"No," the teacher repeated. "You are wrong."
"Aleph is: When your friend asks you for ink, you give it to him.
"Bet is: When your friend asks for ink, you give it to him.
"Gimmel is: When your friend asks for ink, you give it to him."