Espionage or spying is a practice of getting information about an organization, society, or country that is meant to be secret or confidential, without permission. Espionage usually involves having access to where the needed information is stored or to the people that know the information. In wartime, espionage is a war crime. The person who does it is called a "spy" or more vaguely an "agent". A double agent is one who trades information to both sides, and is (without their knowledge) being employed by both sides.
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Painting of French spy captured during the Franco-Prussian War
Espionage Facts for Kids. Kiddle Encyclopedia.