The shell of a hatchling turtle is flexible, because it had to curl up inside the egg. The flexibility comes from bands of soft material (sutures) where the bones join. As the hatchling grows, the soft material stiffens or disappears, and the edges of the bones get jagged like interlocking teeth, making the shell rigid. The scales of material on the top of the shell (scutes), which are made of keratin like fingernails, do not grow. Instead, new scutes are formed under the old ones. Because a turtle grows faster during warm seasons than cooler ones, the scutes form growth rings like a tree that can be counted to estimate age. Depending on the species, the count is not necessarily reliable, since many turtles shed off old scutes. Regardless, turtles are famous for their long lives, with many species living more than 100 years.