汽油
qì yoú · HSK 4
gasoline
⛽ 汽 (steam/vapor) + 油 (oil) = vapor-oil = gasoline
汽 means steam or vapor (the 氵water radical + 气 air/gas), and 油 means oil. Together: the volatile vapor-oil that fuels engines. Compare 柴油 (chái yóu) diesel, literally firewood-oil.
油 — water radical 氵 + 由 (yóu, phonetic)
油 is a phono-semantic compound. The 氵 radical signals a liquid, while 由 supplies the sound. Originally referring to a river name, it was extended to mean oil or fat — any thick, flowing liquid — which became its dominant modern sense.