The lawnmower one isn't funny. That happened to a good friend of mine when she was a little girl, and she's lived with prosthetic legs since then. So don't laugh at that, please because she is such a happy energetic girl that it would break my heart.
You Dont Say, AHAHAHAHAHAHHAAH! I'm not a fan of this kind of humor (it think it gets annoying in real life unless everyone's in the mood), but the "You don't say" Nick Cage meme has got to be the funniest meme ever. I couldn't stop laughing.