Rambam - Sefer HaMitzvos: Positive Mitzvah 98: Positive Mitzvah 98

Positive Mitzvah 98: Impurity of Food and Drink
Leviticus 11:34 "Of all the food which may be eaten...and all drink that may be drunk"

Food and drink can become impure if they have come in contact with a source of impurity.

The Tzemach Tzedek once told someone to become an onion. This was a person who had been travelling from town to town spreading Chassidic teachings. But now he had stopped. All the fuss over himself and his talents, he claimed, had been feeding his ego. Now he had to work with himself to diminish that negative aspect of his character. The Tzemach Tzedek showed no pity. "An onion should become of you!" he cried, "But teach others what you must teach them!"

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Why an onion? An onion is thrown in the pot not so that we may eat onion, but only to give flavor to the chicken and the broth. Like the onion, you sometimes need to sacrifice your own personal growth so that others may grow. And that is growth.

From: Bringing Heaven Down to Earth by Tzvi Freeman - tzvif@aol.com