Street magicians love this trick because it's easy to do and results in a great visual effect. They'll pick up a paper cup, hold it out in front of them, and then move their hands away. It looks like the cup is left floating in mid-air.
There's a reason they always use a paper cup for this - all they've really done is shove their thumb through the back of it so they can hold it in place. That's also why they very quickly dispose of the cup!