You are provided with three watch glasses containing milk, petrol and mustard oil, respectively. Suppose you bring a burning candle near these materials one by one, which material(s) will catch fire instantly and why?

The watch glass containing petrol will catch fire the most easily because, petrol is an inflammable substance and its ignition temperature is also low. On the other hand, milk and mustard oil is not an inflammable substance. A substance is said to be inflammable if it has an ability to catch fire very easily.