Why shouldn't you tell a pigeon a secret

You should never tell a pigeon a secret because the police may know your secret soon after you tell it to the pigeon.

Explanation:

The term 'pigeon' is used to refer to the secret informers of the police in slang/informal language. To be specific, the police informers are known as 'stool pigeons'. The word 'stool' comes from an old French word 'estale'. In medieval Europe, estale used to be a bird that was used to lure other birds to come and get caught in the net.