Fossils are millions of years old. They have been pressed underground or underwater for a very long time. Usually, they are in places where no air can get to them. They are very, very fragile and can break into dust if they are not treated carefully. The skeletons we see in museums today are not usually the real bones found in the ground. The bones would be too heavy to stand up on top of one another, especially if they were stuck together with wire and metal rods. They are also too fragile to survive the rebuilding process. The original bones are kept safe in the laboratory to be studied. When a paleontologist wants to create a skeleton for a museum, he will usually make casts. It takes a long, long time to make casts of all the bones for a whole dinosaur skeleton.
The average dinosaur skeleton is made up of around 200 bones. Each bone is cast separately and then painted to look like the original. When the bones are found, they are rarely in perfect shape. Most of the time, they are broken and scraped and missing some pieces. That’s why you may see some skeletons in museums that are two different colors: one color is the cast of the real bone, and the other is the rebuilt part that paleontologists had to create themselves. Once each bone is re-created, the skeleton is put together like a giant jigsaw puzzle: one piece at a time, and very carefully.