Why would you tell the poor kitty in the picture not to 'cry over spilled milk?' Well, the carton of milk has tipped over and the milk has spilled out. Waaaa! Waaa! Yes, it's very sad but does it help? Herein lies the meaning of this idiom:
Cry over spilled milk: to be upset, worried or unhappy about something that cannot be changed.
Sometimes the best you can do is to stop crying and feeling sad and do the best you can with a situation (e.g., drink the milk off the floor).
This idiom is usually used in a negative form with "don't" or "there's no use."
This is because we are referring to something that has already happened and cannot be changed. The phrase "there's no use" means that trying to do something won't help in this situation (so don't bother to try and change it).
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Writing your own sample sentences will help you practice your English and remember this idiom in the future. When was the last time you cried over spilled milk? Do you know someone who often cries over spilled milk. Do you have this idiom in your language?