When we were kids we were all told about Goldilocks and the Three Bears. Goldilocks tried the Father Bear’s porridge and that was too hot for her. She then tried the Mother Bear’s porridge and that was too cold for her, but when she tried the Baby Bear’s porridge that was just right. Inspired by this, physicists often use Goldilocks to indicate something that is just right. The most common example of this is a Goldilocks planet.

A Goldilocks planet is one which is at a distance from its star so that the planet is just right for life, as it is on Earth. Life on Earth relies completely on water, all living cells run on biochemistry occurring in water. If a planet is too far from its star then liquid water cannot exist, the temperature is always less than 0C and there is only ice. If it is too close to the star water boils and there is only steam. A Goldilocks planet is neither too cold nor too hot.

But the Goldilocks idea is more general than planets. It is used whenever something needs to be just right. This includes something I work on. Crystals, like salt or ice, or chocolate, start to form on surfaces. Together with PhD students here and with colleagues in London, Cambridge and Amsterdam, I’ve shown that the best place for crystals to begin is in pits in a surface. The surface could be anything, from grain of sand to that of a glass beaker.

The starting crystal is very sensitive to the exact size and shape of the pit in the surface, and so like Goldilock’s porridge the pit must be just right. This size is only around a millionth of centimetre across so it is far too small to be seen by our eyes.

The small size of the pits means that a rough surface of even a small particle, say of a grain of sand, will have millions or more of these pits. Only one pit needs to be just right for a crystal to start to form on the grain of sand. Of course with a million tiny pits to choose from, the odds are good that one will be just right, and a crystal will start to grow.