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Water drop impact. High-speed photograph of a water drop impacting on a pool of water, showing secondary drop formation. After the water drop impacts to form a crater, the surface of the pool collapses with inward-rushing water colliding and rising to form a column. Along the column, water surface tension effects create secondary drops. Circular waves ripple out from the centre of impact. (KEYSTONE/SCIENCE PHOTO LIBRARY/ADAM HART-DAVIS)