I present to you a pair of videos from the upcoming Sonic Generations. To appreciate what Sega is doing with this new Sonic game, you'll need to watch them both.
The game takes levels from three eras of Sonic games — classic, Dreamcast and modern era — and lets players run through them either as skinny modern Sonic or pudgy original Sonic. Modern Sonic's version of the level plays out in three dimensions. Classic's version is a sidescroller.
What I've got for you here is, first, modern Sonic running through a re-mix of the Dreamcast's Sonic Adventure 2's City Escape level. The second video is classic Sonic running through a side-scrolling version of the same level.
The man on the controls is Sega Sonic master Aaron Webber. He's playing the Xbox 360 version. The game is also coming to PlayStation 3 and, with a different level set, to Nintendo 3DS.
Look for more about Generations later today.