system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How excess energy is stored as fat and what exactly does "burning fat" entail?
gpt
Say you eat some bread. The bread is digested into sugar in the stomach, then absorbed in the small intestine. Sugar is the body's main energy source. After being absorbed, the sugar circulates in the blood stream. It goes to every single cell that needs energy. If there is leftover sugar, it goes to the liver. The liver turns the sugar, which is also called glucose, into a short term storage form called glycogen. Once the liver has topped up on glycogen, it sends the rest of the available sugar through the blood to special fat cells called adipose cells. These cells absorb the sugar turn it into fat. 

A few caveats. Technically all cells can turn glucose into an intermediate molecule called acetyl-CoA and then turn that it into fatty acids, but it's the adipose cells that store the fatty acids as long term fat. Furthermore, this fatty acid creation mostly happens in the liver and fat cells. Also, two more caveats. Next, all this stuff happens at the same time. Finally, the liver doesn't send anything anywhere becuase it doesn't have decision making powers. It just sort of happens based on the biochemistry of the cells involved.

Oh, as for burning fat. When your body is low on energy, it tells the adipose cells to release fatty acids into the blood. The fatty acids are then "burned" for energy in most of the different cells of your body, (but especially heart cells and muscle cells.) The reason it's called burning is because it's actually the same chemistry as setting a piece of paper on fire, but slowed down into many steps. The energy in sugar and fat is stored in a chemical bond between carbon and oxygen. When you break that bond, you release energy. If your body does it slowly, it can turn it into energy. If you set it on fire, you do it quickly. The original way to see how many calories are in a food was to set it on fire and see how much the temperature changed.