Secondary Flow — the Tea-Leaf Paradox Stir a cup and the leaves gather in a neat pile at the centre — the opposite of what “spun outward” suggests. This is a cross-section of the cup. The swirl is fast in the middle, slow in the thin layer on the bottom (friction). The fast core needs a strong inward pressure to hold it in a circle; the slow bottom layer feels that same pressure but has too little spin to resist it, so it is pushed inward — dragging the leaves with it. The journey: a secondary circulation (in along the floor, up the axis, out at the top, down the walls) rides on top of the main swirl wherever centrifugal force and pressure fall out of balance — the same cell that scours the inside of every river bend.