Answer: burning and melting
Combustion (burning of molecules) is an exothermic oxidation-reduction reaction. When the combustion is intense, it results in a flame or an explosion (deflagration or detonation if the flame front exceeds the speed of sound).
This chemical reaction is a recombination of molecules. It goes through an intermediate step during which the molecules are "destructured" but not yet recombined; these are called radicals and are very reactive. In the case of combustion, the radicals are created by chemical bond breakage due to thermal energy, and they will be able to act on the molecules of the product (releasing other radicals) and thus generating a chain reaction that will continue as long as the following two conditions are met: presence of fuel and oxidizer.
Digestion is a food processing that is ingested into smaller particles that can or can not be assimilated by the body.
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