a spoonful of salt的用法和样例：
- She put a spoonful of salt in the soup.
- A spoon of salt in a lake is almost unnoticed.
- The cook threw in a spoonful of salt as she stirred soup.
- She scooped up a spoonful of salt and threw into the pot.
- A spoon of salt in a glass of watermakes the water undrinkable.