How do they calculate how many calories something has in it like for example how did they find out how many calories are in sugar or something like that?

the way that they find calories is by setting the object a flame and however many degrees celcius it raises 1 cubic centiliter of water, is the amount of calories. for instance if we burned a marshmallow under 3 centiliters of water and it raised 600 degrees,

then u would divide by how many centiliters there were and so the calories would be 200.