启 qǐ to open

怎么记住“启”: 户 door + 口 mouth = to open. The dentist steps through the door, smiles at the scared boy and asks him very gently to open his mouth.

部件结构

会意 · Meaning + meaning

Two or more parts whose meanings together make the new meaning.

启 = 户 door + 口 mouth

Gushi 故事

The dentist steps through the door, smiles at the scared boy and asks him very gently to open his mouth.

It began as a hand opening a door; a mouth was added later, and this form keeps only the door and the mouth.

Bound morpheme 'to open / set in motion': 启动 (qǐdòng) start up (a system, project), 开启 (kāiqǐ) initiate, 启用 (qǐyòng) put into use.
Also 'to enlighten': 启发 (qǐfā) inspire, 启蒙 (qǐméng) enlighten the young, 启示 (qǐshì) revelation.
In notices 启事 (qǐshì) means a public announcement: note it is 事, not 示.