Why Do We Put a Spoon in a Glass Before Pouring in Hot Water?
Putting a spoon in a drinking glass before pouring in hot water helps to prevent the glass from breaking. Glass expands, or swells, when it becomes hot. When you pour boiling water into a cold glass, the glass expands very quickly, but unevenly.
This uneven “strain” is sometimes so strong and sudden, that the brittle glass cracks right through. A spoon in the glass sprays the hot water against the sides, which allows the glass to heat more evenly. The water is also cooled slightly by this contact with the sides, as well as by the heat absorbed by the spoon.
This phenomenon which causes glass to break o shatter when we pour boiling water into it is called thermal shock. We can prevent cups and mugs from breaking by using generic glass for hot liquids; stick to ceramics or glass that we know to be heat-resistant.
Properly fired ceramics can handle the shift from room temperature to boiling water just fine, but a big enough shock will shatter just about anything.