If a cork is put into a glass of water, the cork will almost always drift to the side of the glass. There is one simple way, however, to get the cork to float in the centre of the glass (the horizontal centre, not the vertical). What is it? Water, the glass, and the cork are all that is required.
The reason that a cork drifts to the side of a glass is that it floats to the highest point. Since water “clings” to the glass, the highest point is around the edge of the water. To get the cork to float in the middle of the glass, all you have to do is fill the glass as much as possible. The water will form a convex shape above the glass, with the highest point at its centre. This is where the cork will settle.