Red/Blue: It is vulnerable to attack because its shell is soft, exposing its weak and tender body.
Gold/Silver: Inside the shell, it is soft and weak as it prepares to evolve. It stays motionless in the shell.
It prepares for evolution by hardening its shell as much as possible to protect its soft body.

This is its pre-evolved form. At this stage, it can only harden, so it remains motionless to avoid attack.

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