火 pictures rising flames — a central plume with two flickering tongues at the sides. Identical across 繁體 / 新字体 / 简体. Like 水, 火 has a critical positional variant: when it appears at the bottom of a character, it flattens into 灬 (four dots, called the "fire-foot" radical). Once you spot 灬 you can predict heating or cooking: 熱 (hot), 煮 (boil), 焦 (scorched), 烈 (intense), 然 (originally "burn", now mostly grammatical). The four dots are not water — they are fire seen from above.

Mandarin: huǒ, dipping 3rd tone. 火 alone is fire; 火车 (huǒchē, "fire vehicle") is a train, named in the steam-engine era and never updated; 火山 (huǒshān) is volcano; 火锅 (huǒguō, "fire pot") is hot pot, the communal Sichuan meal.

Japanese: on-reading カ (ka) in 火曜日 (kayōbi, Tuesday), 火災 (kasai, fire/blaze), 花火 (hanabi, fireworks — literally "flower-fire"). Kun-reading ひ (hi) — same syllable as 日 (sun), so context disambiguates. Note the planetary week: Tuesday is "fire day", named for Mars (the Mars/fire planet) in the East Asian system that parallels the Western Tuesday-from-Tiu.

Memory aid: a vertical flame with two side sparks. If you ever see four small dots running across the bottom of a character, mentally translate "fire underneath".