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I was boiling chunks of potatoes when I noticed that they were surrounded by a correspondingly shaped outline on the water’s surface (see photo). The pan had been on the stove for only a couple of minutes so the water was not yet boiling. What causes the lines on the surface to form and why do they follow the outlines of the potato pieces?

• The chunks of potato are less solid than they seem. Air passes through microscopic channels between the cell walls. The heat of the water expands the air, forcing out tiny bubbles containing air plus warm water vapour.

There is little difference between the bubbles emitted from the various sliced faces, but these potatoes have been cut into sections with more or less flat faces at right angles, so some of the faces are roughly vertical as the chunks rest on the bottom of the pan. On a microscopic scale, bubbles on vertical faces are held in place by damaged cell walls, so they grow larger before they escape. Bubbles from horizontal faces generally are smaller and less conspicuous because they pop up more easily.

More importantly, the bubbles from each entire vertical face rise and concentrate visibly along a common line, unlike those from horizontal faces that scatter inconspicuously over the whole surface of the water.

Furthermore, bubbles floating on water attract each other, so a line of bubbles clinging together is conserved, remaining pinned to where new bubbles keep rising. Individual bubbles over a horizontal face drift arbitrarily across the surface until they fall into line.

Jon Richfield, Somerset West, South Africa

• The potatoes had probably not been washed between being cut and placed in the pan. As the water warms, a combination of tiny air bubbles and a thin film of starch on all the cut faces forms a fine foam which rises to the surface.

The amount involved is small, so the foam rising from the flat horizontal faces of the potatoes is not distinguishable. But bubbles rising from the vertical faces have arrived at the surface in a single line and show up more easily.

Bryn Glover, Cracoe, North Yorkshire, UK