There was a 1993 episode of Seinfeld that focused on a specific area of hygiene and entered a phrase into the common lexicon. It is now common to hear people speak of "double-dipping" and its lack of social acceptability. Here is a transcript of that brief exchange (from here):
[George, attending a wake, takes a large tortilla chip, dips it into a bowl of what appears to be sour cream, takes a bite, dips it into the bowl again, and then eats the remainder of the chip.]The point is that we should be careful about spreading our germs and considerate of fellow eaters. There is an interesting parallel in a talmudic passage, that not only makes this point but does so in a fairly humorous, and a little gross, way.
Timmy: What are you doing?
George Costanza: What?
Timmy: Did, did you just double dip that chip?
George Costanza: Excuse me?
Timmy: You double dipped a chip!
George Costanza: Double dipped? What, what, what are you talking about?
Timmy: You dipped a chip. You took a bite. And you dipped again.
George Costanza: So?
Timmy: That's like putting your whole mouth right in the dip. From now on, when you take a chip, just take one dip and end it.
Here it is in Nedarim 49b:
R. Yossi and R. Yehudah. One was eating porridge with his hands and the other [from the same bowl] with [a utensil fashioned out of] tree bark. The one eating with bark said to the one eating with his hands: "Until when will you keep feeding me your excrement?" The one eating with his hands said to the one eating with bark: "Until when will you keep feeding me your saliva?"These two rabbis were eating from the same bowl. One pointed out to his fellow that his hands were dirty, jokingly referring to them as being full of excrement (Rashi says that there was dirt under his fingernails). The other retorted that every time he put his utensil back in the bowl, i.e. double dipped, he was putting his saliva back in. I can picture this exchange happening and find it hilarious. But the Talmud's point is simple: Be hygienic and be considerate. And, clearly, take just one dip and then end it.