西瓜不知柠檬酸柠檬不知西瓜甜
[xī guā bù zhī níng méng suān níng méng bù zhī xī guā tián]
The meaning here is metaphorical, implying mutual lack of understanding or empathy between two individuals with different experiences or backgrounds. Just as watermelon cannot know the sourness of lemon, nor can lemon know the sweetness of watermelon.