How did French pilot make fun of German soldiers during WW1?

During one of the most violent fights with German army on first April 1915; French pilot dropped a huge bomb. German soldiers in the expectation of the explosion ran in different directions, however they didn't hear any explosion. Instead of a bomb there was a giant ball with mark: "Happy fools day!"




SOURCE: http://www.museumofhoaxes.com/hoax/aprilfool/P20/
IMAGE: www.militaryfactory.com