A Very Hyper Cube
The Movie: When Loki arrived in the JDEM facility, Nick Fury tried to hide the Tesseract in a suitcase, burning his fingers slightly in the process. Bruce Banner later estimates that Loki heated the Tesseract to 120 million Kelvin in order to break the "Coulomb barrier" allowing him to teleport to Earth.
Real Life: Yeah, if Fury had touched a 120 million Kelvin cube, he would have had more to deal with than finger burns. In fact, an object of that temperature, even a small cube like the Tesseract, would probably make Earth go the way of Alderaan. Consider that the belly of the Sun is only 16 million Kelvin and a suitcase of that stuff would be as devastating as an atom bomb. Also, we're not sure where Dr. Selvig got his degree, because his investigative technique appeared to be poking the Tesseract in random places.