The performer exhibits a goblet which is nearly full of ink. He announces that he's going to transform the ink into something else — milk, perhaps. He dips a business card into the ink and shows that it's become stained. He also spoons up a little of the ink and pours it into a transparent bowl to reinforce the impression. He borrows a ring from someone in the audience which, he says, is necessary because all his magic rings have lost their effect (or some similar patter). Clumsily, while waving the ring over the goblet he drops it in. He swears and asks his assistant to get a rag or something so he can pluck it out without getting the ink on his fingers. He covers the goblet and dips into it to retrieve the ring, but when he removes the rag the goblet now contains clear water with a fish or two swimming around in it! He professes amazement!
The goblet actually contains the water and fish to begin with. But the inside of the goblet is completely lined with black rubber cloth; a black thread is attached to this, a few inches long, with a small cork or other "grab-able" thing. The performer palms the ring and drops something inconspicuous into the glass — say a small pebble — to make sure there's an appropriate sound. When he places the rag over the goblet, he grabs the cork and uses it to remove the rubber cloth, concealed with the folds of the rag, of course; then he un-palms the ring to return it.
Obviously the thread and cork must at all times be behind the goblet or otherwise disguised so that the audience doesn't see them. When the ring is requested, the assistant should go to the front of the stage to get it; an accomplice in the audience could be used, of course; but otherwise the audience member mustn't be allowed close enough to the goblet to see what's happening. The performer can choreograph it as he sees fit. The card is prepared: one vertical edge is actually already ink-stained but this can be concealed by manipulation. The spoon that dips up the ink must be prepared: a few particles of aniline black must be fixed into its bowl by breathing on them.