密封 mìfēng to seal up
How to remember 密封: 密 secret + 封 envelope = to seal up.
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How it is used
- Airtight/watertight sealing - 密封的罐子 (mìfēng de guànzi) a hermetically sealed jar; 请密封保存 (qǐng mìfēng bǎocún) keep sealed.
- The 密 stresses tightness against air or moisture; contrast plain 封 (fēng, to close/seal an envelope) which just means shut, not airtight.
- Common on packaging, food storage, and technical contexts.