system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do we know when our bodies are burning fat? Is it when we’re hungry and our tummies have the rumblies?
gpt
We don't "know" ourselves. Hunger doesn't necessarily mean we're burning fat, it just means our stomachs are empty. We will first use up other resources, like ketones, and then we'll move on to fat burning, but that will be some time after we start to feel hungry, generally speaking.