During the two-parter premiere of Season 3 (The Crystal Empire), the show had 4 ads about GAK, a slimy modeling clay from Mattel that had been out of the air since it started in 1992 and now returned licensed under Nickelodeon. I't like an ultra-stretchy Play-Doh that can do practically everything the image above shows (I doubt a bit about its train-stopping capabilities). Since then, it became part of the brony community due to that, and now there a couple of rumors that say that Pinkie might be made of GAK.
haha, I'd honestly expect Pinkie to do one of these things on the show, like Courage the Cowardly Dog! After all, she's got the physics-defying personality to do it! XD