Universe is mainly made up of 3 types of matter:
so most of the universe is made of three types of matter i.e
Dark energy,Dark matter and Normal matter
when we burn a piece of paper wha happens
1.The atoms of molecules combine to release energy which is in the form of light and heat
2.The carbon and hydrogen in the paper combine with the oxygen in the air to form co2 and so here the solid elements carbon and hydrogen converts in to gaseous state by reacting with oxygen and released in the atmosphere
3.Thus we think that the mass or energy of the paper or anything which is burnt is reduced but actuall not.
How is the question?
Thus its been said that the energy and mass of the bodies are same i.e the energy can neither be created nor destroyed but converted from one state to other
on the other hand if you burn a metal like steel or copper it reacts with the oxygen in the air and the oxygen in the air sticks to the metal and gains mass