1.
to open one's mouth (to speak, yawn, or receive food, medicine, or a dental exam)
The basic, literal sense: to physically open the mouth. Commonly heard at the dentist (口を大きく開けてください), when feeding a baby or an animal, or when describing someone yawning or singing. Note: for the sense 'to start talking; to speak up', Japanese usually prefers 口を開く with the reading ひらく.
口を開けてください。
Please open your mouth.
赤ちゃんが口を開けてあくびをした。
The baby opened its mouth and yawned.
歯医者さんに「もう少し大きく口を開けて」と言われて、必死であごを広げた。
When the dentist said 'Please open your mouth a little wider,' I spread my jaws open for all I was worth.
2.
to open (a bottle, package, or other container); to unseal
By extension from the 'mouth' metaphor for openings: to remove the cap or seal of a bottle, jar, bag, or similar container, often for the first time. Common with foods, drinks, cosmetics, and medicines.
ワインの口を開ける。
To open a bottle of wine.
新しい醤油の口を開けたら、香りが立った。
When I opened the new bottle of soy sauce, the aroma wafted up.
お土産にもらったジャムの瓶の口を開けたら、甘い香りが部屋いっぱいに広がった。
When I opened the jar of jam that I had received as a souvenir, a sweet aroma filled the entire room.
The construction is noun 口 + を + verb 開ける. 口 here refers literally to the mouth (sense 1) or to the opening/mouth of a container (sense 2); 開ける is the transitive verb 'to open'.
USAGE:
- Sense 1 is nearly always concrete and physical. The metaphorical 'open one's mouth = start speaking' sense is normally carried by 口を開く (same kanji, but read ひらく). Beginning learners often conflate them.
- For the intransitive counterpart (a mouth/container opens by itself or is open), use 口が開く or the state form 口が開いている.
- In sense 2, 口を切る is a near-synonym, especially for unsealing a new package or bottle for the very first time.
COMMON COLLOCATIONS:
- 口を大きく開ける: to open one's mouth wide
- 口を開けて笑う: to open one's mouth and laugh
- あくびをして口を開ける: to open one's mouth in a yawn
- ワインの口を開ける: to open a bottle of wine
- 瓶の口を開ける: to open the mouth of a bottle
- 袋の口を開ける: to open the mouth of a bag
SIMILAR WORDS:
- 口を開く: to open one's mouth (and speak); to break one's silence — figurative use, focused on speech.
- 口を切る: to be the first to speak; also to unseal a new container — overlaps with sense 2 when referring to opening something for the first time.
- 口を閉じる / 閉める: to close one's mouth — the antonym.
- 開ける: to open — the general transitive verb; combines freely with many objects (door, window, box).