When the teaspoon causes the tea in a glass to rotate, a distribution of pressures at the bottom of the glass is set up which increases from the centre of the bottom towards the sides (since the level of tea is higher at the sides than in the middle). After the teaspoon is removed, the pressure at the bottom will be gradually evened out. This leads to the formation of currents from sides to centre and it is these which gather the tea-leaves to the middle of the glass's bottom.