Alt balloon meaning...

Do you know the origin of the saying "Once the Balloon Has Gone Up"?

"Once the balloon has gone up means when trouble is here, or in a period of trouble. The term once the balloon has gone up derives from the First World War. Whenever enemy activity was expected, observations balloons would be released to monitor the enemy troop movements. As a result, the raising of these balloons, which were visible to all, soon became a sign of pending enemy action."

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