A grandfather is telling his grandson war stories.

"At the end of World War I, I was awarded for my bravery after saving a group of my men," the grandfather says. "You see, we were fighting in northern France and one of our enemies threw a grenade at us. I managed to pick it up and throw it away before it exploded. So right after the war ended, a General gave me a sword, engraved with the words 'Awarded for Bravery and Valor, A True Hero, World War I'."

The grandson thinks about the story for a minute and then says "Grandpa, that story can't be true!" How did the grandson know?