臼

1. Origins, structure, and basic meaning

• Structure: 臼(うす) means a tool such as a mortar or millstone used to pound or grind grain.
• Basic usage: It is used for a mortar for pounding rice cakes or a tool for grinding grain.

2. Common pitfalls and similar expressions

• Different actions for the tool
  • 餅をついた means pounding rice cakes.
  • 穀物をひく means grinding grain, showing how the tool in the example is used.