热

热 (rè) means 'hot' — referring to temperature, whether of the weather, an object, food, or a person who feels warm. It works exactly like an English adjective but has one key Chinese-grammar feature: predicate adjectives don't need a verb, but they DO usually need an intensifier like 很 (hěn). So 'It's hot' is 很热 (hěn rè, literally 'very hot'), not 是热 — using 是 (to be) before an adjective is wrong. Without 很, a bare 热 implies contrast ('hot, not cold'). Beyond temperature, 热 has two important extended meanings: 'popular / trending' (热点 'hot topic', 热门 'popular') and 'warm-hearted / passionate' (热情 'warm and enthusiastic', 热心 'kind and helpful'). The opposite, 冷 (lěng), is its direct counterpart on the temperature scale.

Memory hook: The four dots at the bottom of 热 are the 'fire' radical (灬) — anything with that radical relates to heat or burning.