A hard surface that covers an animal is its shell. Sea urchins, turtles, and lobsters all have protective shells. The largest turtle shell is nearly 10 feet wide!
Most shells are made of calcium carbonate, and they grow along with the animal. Bird embryos are nestled inside shells, and nuts have shells too. Other uses for shell come from the biological meaning. A rifle shell has a shell-like metal casing. An empty structure, with only walls, is the shell of a building. And to "come out of your shell," means to emerge, like a turtle, from your shyness.
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