Wine

 Someone called on a friend with a very expensive bottle of the most wonderful red wine. His friend produced a couple of filthy wine glasses, rubbed his hands, and said Pour away! After an awkward brief pause the visitor unexpectedly got up and said Sorry no, I’ve got to go - and away he went and took his wine with him. The friend, a little perplexed, shrugged and got up and grabbed some cheap bottle from somewhere and poured from that out into his dirty glass instead.