Take two paper cups. Keep one cup empty and in other pour some water to make it wet. Now hold both of these cups on two different lighted candles. You will observe the cup which is empty catches fire earlier than the other one. And the other wet cup will catch fire after some time.
Lets understand what is the science behind this observation.
1. When you keep empty cup in candle flame its temperature rises, reaches its ignition temperature and catches fire.
2. In the case of wet paper cup, When you place it on the flame of candle, firstly it uses all the heat to evaporate water from it. Till then time temperature of cup do not rises to its ignition temperature, When whole of the water from it gets evaporate, heat giving by flame of candle used up in rising temperature of cup. Now its temperature reaches to its ignition temperature and it catches fire.