殻

1. Origins, structure, and basic meaning

• Structure: 殻, read から, is a single noun written for the hard outer casing of an egg, shellfish, or seed.
• Basic usage: It denotes a hard outer covering around contents, as in 卵の殻 or 貝の殻.

2. Common pitfalls and similar expressions

• Distinguishing meanings
  • 殻 suits a hard shell or an empty casing, while 皮 is normally used for a thin or soft fruit skin.