Swirl water in a cup, and the water will move to the outer edge of the cup. Sometimes, when you swirl hard enough, it will actually start "climbing" the walls of the cup, as the water collects at the outer edge. Now take a cup of tea with a few tea leaves in it and stir it with a spoon. The tea leaves collect in the middle of the cup, proving tea leaves really are magic.

Or maybe not. Maybe there's something else going on, and Albert Einstein is just the guy to figure it out. Einstein determined that, just where the liquid meets the glass, there's a little drag on the flow of the tea. The tea right at the sides of the cup, and more importantly, all along the bottom, will be going less quickly than the rest of the liquid in the cup. This is important, as all that water pushing up against the sides of the cup also establishes a pressure gradient, with higher pressure towards the outside of the cup. The slow-moving water is displaced. When it hits the sides of the cup, it drops downwards, and when it gets to the bottom of the cup, and the relatively slow-moving water there, it's forced inwards. Once it gets to the middle it's forced upwards, and the cycle continues again in a kind of invisible roll inside the swirling tea. The only visible signs are the tea leaves themselves. They are too heavy to be lifted up by that upward flow in the middle of the cup, but they do get caught up in that roll. So they're herded towards the middle of the cup, defying centrifugal force. Thanks, Einstein!