There used to be two great rabbis, Hillel and Shamai. Shamai was very strict and literal in his interpretations of the Torah, whereas Hillel was a lot more lenient and easy-going. They each had hundreds of followers and students who would regularly get into heated arguments.

One day a man came to Rabbi Shamai and asked him "teach me the whole Torah on one leg" (meaning, teach it to me quickly, in the time I could stand to be on one leg). Shamai became very angry and had his students chase the man out of the house.

So the man went to Rabbi Hillel and asked him "teach me the whole Torah on one leg". Hillel looked at him curiously, and then thought for a long time. Eventually he took a piece of paper an wrote on it "love your fellow like yourself". He gave this note to the man and said "this is the whole Torah on one leg".