Bomb, duck, same thing

My in-laws took my nephews, nearly 5 and 6.5 at the time, up to London a month or two ago. They went to the Science Museum, and had their bags checked by security as they entered. The security guard explained that he was looking for anything that could be dangerous.
"You mean like a bomb?" asked one boy.
And his younger brother pipes up, "You mean like a duck?"