In my fan-fics, Casper can detach his head, but is careful not to allow* it to be stolen. If he detaches it himself, it doesn't hurt. Yet if removed against his will, it hurts !
* Owing to a mental quirk, I have at times used adjectives as verbs - to possible his head to be stolen.
In my fan-fics, Casper can detach his head, but is careful not to allow* it to be stolen. If he deta