Several parts combine into one character.

科 is a compound ideograph of 禾 (grain) and 斗 (a dipper for measuring grain) — measuring rice (禾) with a dipper (斗) to grade it. From measuring grain to sort it by quality grew "grade, division, subject, department."

Korean hanja-sound "gwa": 科目 (gwamok, school subject), 科學 (gwahak, science), 外科 (oegwa, surgery), 內科 (naegwa, internal medicine), 學科 (hakgwa, academic department), 敎科書 (gyogwaseo, textbook), 科擧 (gwageo, the imperial civil service exam). The old civil service exam 科擧 (gwageo) — "to select by grading" — uses this character.

Mandarin: kē, 1st tone. 科学 (kēxué, science), 科技 (kējì, science and technology), 学科 (xuékē, discipline), 外科 (wàikē, surgery), 百科全书 (bǎikē quánshū, encyclopedia). 科技 (kējì) is a core word in daily modern Chinese.

Japanese: on-reading カ (ka) — there is no kun-reading (this is an on-only character). 科学 (kagaku, science), 教科書 (kyōkasho, textbook), 外科 (geka, surgery), 学科 (gakka, department), 科目 (kamoku, subject). Surgery is read geka, pairing with the 外 (ge) seen earlier.

Memory aid: rice (禾) measured by a dipper (斗) — sorting grain into grades, hence subject and division.