FACT: In the United Kingdom and Ireland we measure our feet. This is not a particularly abnormal thing, after all most people measure their feet, the difference is the shoe sizes. Some use inches, others use centimetres. We do it a different way.
In Britain and Ireland the unit is barleycorns. This is approximately ⅓ of an inch. That’s it, the difference between a size 4 and a size 5 is one barleycorn. Although this unit is bizarre in nature it has spread from the UK and is also the basis of the US. So now you know the difference between the sizes, the answer is corn.
Note: a size 1 shoe is not 1 barleycorn in length ie ⅓ of an inch, it has to do with the length of the ‘last,’ but that is all very boring I assure you. Size 1 is just a calculated size 0 + 1 handsome barleycorn.