system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a vending machine know the difference between a One Dollar bill and something that isn't a One Dollar bill?

Like if I put a 100 Dollar bill in the machine would it know that it's a hundred? 

But couldn't you put anything in a vending machine and still have it work if it resembles money?
gpt
The Treasury uses (among many other security features) a special magnetic ink. There's a head in the bill reader that scans for a specific pattern of magnetic ink, and usually some other optical sensors. 

Coins, on the other hand, are distinguished by using surprisingly accurate measurements of its width, thickness, weight, and number of ridges along the edge.