When we burn food, why is there energy released that can be used to heat up water?

Question

is this to do with heat eneryg??also what is the point in this energy and how does it heat up the water?

Answer 1

Your question seems to fold a couple of terms a bit inaccurately, but ...
If by "we burn food" you mean "when we as people use food" then the energy released is what keeps us alive, normally by the materials being converted to simpler sugars and proteins in digestion that are moved to the muscles, etc., for growth and movement.
if you mean "we burn food to test it by heating water" the reason we can test the material in food is that the total energy available is the same, although in one case it is burned directly to CO2 and H2O while in the body it takes several steps to get the energy to the right place (as above) before it is converted to result products that are eliminated as CO2, urine, etc.