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.

Once a distinguished rabbi complained to the Tzemach Tzedek (the third rebbe of Lubavitch) that nobody cared for his opinion and that his colleagues stepped all over him.
The Tzemach Tzedek replied, "Who told you to spread yourself around so much that wherever anybody steps they must step on you?"

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