My grandmother loved "Tsitron" sweets, which had a bright lemon flavour and a pretty wrapper with lemons on it

My grandmother loved "Tsitron" sweets, which had a bright lemon flavour and a pretty wrapper with lemons on it. But it never occurred to me that the citron is a fruit in its own right; I took it for just another name for the lemon. It was thanks to a painting by Francisco de Zurbarán that I learnt that the citron and the lemon are different species of the genus Citrus.

In Zurbarán's painting of 1633, "Still Life with Lemons, Oranges and a Rose", it is citrons that are depicted.

The canvas itself is called an allegory of the Holy Trinity, and sometimes other Christian symbols are found in it too: the cup of water and the orange blossom may allude to images of the Virgin, while the rose without thorns may imply the Immaculate Conception.

The painting is on display at the Prado Museum until 30 June; it is usually found in the collection of the Norton Simon Museum in the United States.