A man I coach sent me a photo last spring. Not a lift, not a plate of food. A handheld meter with one number glowing on it. 2.1. Underneath he wrote, "Dialed in." He had chased that number for weeks and finally caught it, and I was happy for him. I was also quietly certain it had told him almost nothing about what was happening inside his cells. That gap, between the number a device reports and the work a cell is actually doing, is the whole argument about ketones, and it is where nearly everyone gets lost. Two people can hold up meters reading the same 2.1 and be living in completely different bodies. One fed a fire. The other smothered it. The meter cannot tell them apart. So let me take you inside the cell the slow way, and let the fire teach the rest. Inside almost every cell sit structures called mitochondria, and it is fair, almost literally, to call them furnaces. They take fuel and oxygen and burn the two together to release energy, the way a wood stove turns logs and draft into heat. What comes out is not flame but a molecule called ATP, the cell's spendable currency. The intuition of a fire holds all the way down. A fire cares what you feed it, how fast you feed it, and how much air it can pull. Get any of those wrong and the same fuel that should warm the room fills it with smoke. Ketones are a fuel for that fire. Beta-hydroxybutyrate, or BHB, is the main one. Your liver makes it from fat whenever food runs short, and your heart and brain burn it gladly. The trouble starts the moment someone sells you BHB in a tub. Here is the first thing the meter hides. When you prick your finger, it reads how much BHB is floating in your blood that instant. It does not read how much is being pulled into a furnace and burned. Those are different questions, and confusing them is the original mistake underneath almost all ketone marketing. A reading of 2.1 tells you a certain amount of firewood is stacked in the yard. It says nothing about whether any of it is in the stove, catching, throwing heat. Dump more wood in the yard than the stove could ever take, and the surplus gets hauled off. In the body the haulers are your kidneys. Past a certain blood level you spill ketones into your urine, and the meter counts that lost fuel as a triumph your cells never touch.