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Jurassic Park
Having a wildlife park full of dinosaurs would be a really cool idea if it weren't for a few problems. No, not imperfect security or the possibility of spontaneous lizard sex changes. The problem is that it would be almost impossible to clone the dinosaurs based on DNA pulled from the guts of a 25 million-year-old mosquito. The dinosaur DNA's double helix most certainly would have been broken down into individual chunks, mixing together with whatever else the mosquitoes might have eaten along with some of the insect's own genetic material. Any creature constructed from that mess might be the stuff of nightmares, but probably wouldn't look like a T. Rex.
(source: movies.yahoo)
(source: movies.yahoo)