Final answer:
Water-filled cups do not catch on fire when held in a flame because water has a high heat capacity and can absorb the heat from the flame.
Step-by-step explanation:
When a water-filled cup is held in a flame, the water prevents the cup from catching on fire. This is because water has a high heat capacity, meaning it can absorb a lot of heat before reaching its boiling point and turning into steam. The heat from the flame is transferred to the water, causing the water to absorb the heat and prevent the cup from reaching its ignition temperature.