Why dried coffee leaves a dark ring

Look at a drop of coffee that has dried on the table. It almost never leaves a full, even circle: it leaves a ring, dark at the rim and pale in the middle. It's no accident, it has a name and an explanation.

The edge of the drop stays 'stuck' to the table and doesn't shrink. But it's precisely at the edge that the liquid evaporates fastest. To replace what escapes, a current forms that runs from the centre outwards, and that current drags the coffee particles floating inside all the way to the rim.

When the drop finishes drying, almost all the coloured grains have been carried to the perimeter and settle there. Hence the ring. Physicists study it seriously because it governs how inks, paints and even medical tests dry. A simple coffee stain hides a tiny current at work.