If someone had asked me what strength was before I sat across the back of a dragon... well I might have said strength was in killing dragons. But now? Now strength is doing the unheard of, the unthinkable. Its being willing to stand for what you have found to be true even if everyone else mocks or rejects you. If I'm being honest, Hiccup was the first person to show me what true strength was... is. Not only did he stand up to his dad, the chief of Berk! He stood up to the entire village, and he showed them the truth. To this day he'll say he was scared, but it was his fear that brought out true strength.
