In the ancient Greek times, poison was a very popular murder weapon. So of course, you had to be wary of whatever you were being served when you drank with someone. So how do you reassure your guests that you're giving them a cup of wine and that you're not trying to murder them?

You pour everyone's drinks from the same source. Then, if that's not enough, you clank your cups together so that the liquid spills into each other's cups. This means that if I tried to poison you, then some of the poison also went into my own cup.

This gesture would let people know that there was nothing to worry about and that they were amongst friends. Unless of course, someone had a little bit of antidote handy.