柠檬不甜会酸
[níng méng bù tián huì suān]
Translated as 'If lemon is not sweet, it will be sour.' This suggests that if things aren't perfect (like expecting a sweet flavor from a lemon) then there are bound to be challenges or hardships which are metaphorically described here using sour flavors of life. This can express one's acceptance or humor toward imperfect conditions.