"A Colorado State University physics professor, using dimensions given by Richard Heene, had told sheriff's officials as they were tracking the balloon Thursday that it was plausible for it to lift off with 37-pound Falcon inside.
Once the device landed, sheriff's officials discovered it was made with plastic tarps taped together and covered with aluminum foil, with a utility box made of a very thin piece of plywood, cardboard on the side, held together with string and duct tape, Alderden said.
Using the true dimensions, the professor determined it could not have launched with the boy inside, Alderden said."
http://www.huffingtonpost.com/2009/10/18/balloon-boy-hoax-sheriff-_n_325162.html