How to say "You can't make a silk purse out of a sow's ear" in Spanish (Día 118)

The video introduces a favorite Spanish phrase and its English equivalent, 'You can't make a silk purse out of a sow's ear,' meaning you can't change something into what it's not. Two Spanish translations are provided: 'No se puede pedir peras al olmo' and 'Aunque la mona se vista de seda, mona se queda,' both conveying the idea that things cannot be changed from their nature. The video concludes with a farewell.

How does the proverb about the Elm tree relate to expectations in life?

What is the common theme shared by the proverbs discussed in the text?