To add something glossed over by other (wise excellent) answers: Where does carbon-14 come from if you
Can’t get more while dead
It decays
The answer is the sun! There’s nitrogen in the atmosphere (specifically, nitrogen-14), and it sometimes gets converted into carbon-14 due to radiation from the sun. Living things use the carbon you breathe in to build new cells/parts of their bodies. Dead things stop doing this.
Let me complicate the story more! At long time scales, the amount of nitrogen, carbon, and carbon-14 can vary in the atmosphere (for example, sunspots might change the radiation, or a volcano might release a lot of gasses). So how do we know what ratio of carbon to carbon-14 things started at? The atmosphere is relatively well mixed afaict, and so this happens at a relatively uniform rate. So we just have to find something that we’re really sure about how old it is, and use the ratio of carbon to carbon-14 in that as a baseline. This is where ice cores come in! Ice is quite structural, and we can tell when an ice pack was put down up to the ~decade. So if we find organic things (even small ones) in an ice core sample, we can use the ice to date them and then use their carbon to carbon-14 ratio to date other things.
To add something glossed over by other (wise excellent) answers: Where does carbon-14 come from if you
The answer is the sun! There’s nitrogen in the atmosphere (specifically, nitrogen-14), and it sometimes gets converted into carbon-14 due to radiation from the sun. Living things use the carbon you breathe in to build new cells/parts of their bodies. Dead things stop doing this.
Let me complicate the story more! At long time scales, the amount of nitrogen, carbon, and carbon-14 can vary in the atmosphere (for example, sunspots might change the radiation, or a volcano might release a lot of gasses). So how do we know what ratio of carbon to carbon-14 things started at? The atmosphere is relatively well mixed afaict, and so this happens at a relatively uniform rate. So we just have to find something that we’re really sure about how old it is, and use the ratio of carbon to carbon-14 in that as a baseline. This is where ice cores come in! Ice is quite structural, and we can tell when an ice pack was put down up to the ~decade. So if we find organic things (even small ones) in an ice core sample, we can use the ice to date them and then use their carbon to carbon-14 ratio to date other things.