How does 'the cup trick' done by people on streets in Paris work?

Just came back from a stay in Paris. I noticed a number of men on streets who would have 3 cups and 1 ball, jumble them around rapidly and people would have to guess where the ball was. If they guessed correctly they would have their money doubled. It is however obvious that it is a scam. It's far too easy to... show more Just came back from a stay in Paris. I noticed a number of men on streets who would have 3 cups and 1 ball, jumble them around rapidly and people would have to guess where the ball was. If they guessed correctly they would have their money doubled.

It is however obvious that it is a scam. It's far too easy to guess where the ball is. So how do these tricksters make money? Do they take it and run off?
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