What do you mean by sour grapes instead of stupid?

Only when you can't eat grapes can you say that grapes are sour. I'm not saying I'm stupid, but that sour grapes come from foxes picking grapes in fairy tales. The novel is about a fox who picked up a lot of grapes, but couldn't reach them, saying that the grapes were sour. When the bird next to him heard it, he said that he was not stupid, but sour grapes, which meant laughing at the fox and saying sour grapes.