What is the difference between fire and warm body?
You're really asking about the difference between a physical change and a chemical change.*
In a physical change, the chemical identity of the substance doesn't change.  In a chemical change, it does.
Suppose you take cube of sucrose (table sugar) at room temperature.  If you heat it up, it will become warmer.  But it will still be, chemically, sucrose.  It will just be sucrose at a higher temperature.  Since the chemical identity did not change, it is a physical change:
$$\ce{C12H22O11(s, room\,T)  -&gt; C12H22O11(s, warm\,T)}$$
[$\ce{C12H22O11}$ is the chemical formula for sucrose; &quot;s&quot; means solid, &quot;l&quot; means liquid, &quot;g&quot; means gas.]
If you warm it more (to $170 \,^\circ \text{C}$) it will melt.  Melting is a physical change, because after the cube melts, it still consists of sucrose molecules; though these will now be in the liquid state:
$$\ce{C12H22O11(s)  -&gt; C12H22O11(l)}$$
But suppose you instead flow enough energy into it to break the bonds that hold the sucrose together, causing it to combust, in the present of oxygen, into water and carbon dioxide.  That is what happens when it burns.  This is a chemical change because, as it is happening, the sucrose and oxygen are being changed into new chemicals: water and carbon dioxide:
$$\ce{C12H22O11(s) + 12 O2(g) -&gt; 12CO2(g) + 11 H2O(l)}$$
This chemical change releases energy, which (in this case) takes the form of heat and light (which is why it feels hot, and you can see a flame).
In sum, a warming body is undergoing a physical change.  A burning body is undergoing a chemical change.
[*Yes, some might take issue with this distinction, but it is used in all first-year chemistry textbooks I've seen, indicating there is a consensus among educators in this field that this is a useful distinction for beginning students to learn.]