火 is a picture of a flame — a central blaze with sparks flying off on either side. You can still see the fire dancing in the shape: the two outer strokes are sparks, the middle is the heart of the flame.
火 is one of the Five Elements (五行: wood, fire, earth, metal, water) that traditional Chinese thought used to explain everything from medicine to fortune. 火 also means anger ('上火' = to get inflamed/angry, also 'to have too much internal heat' in Chinese medicine) and popularity — a hit show is '很火' (very fire = super hot right now).
A flame with sparks flying off both sides — FIRE.
When 火 sits at the bottom of a character it shrinks to four dots: 灬. So 热 (hot) and 点 (a dot / to order food) are literally sitting on a tiny fire.