Why does a US $20 bill have a vertical strip on it when held to light?
Paper money has counterfeit measures to ensure it is very difficult to duplicate/fake
Below is an instruction that describes a task, paired with an input that provides further context. Write a response that appropriately completes the request.
### Instruction: Why does a US $20 bill have a vertical strip on it when held to light?
### Input: 
### Response: Paper money has counterfeit measures to ensure it is very difficult to duplicate/fake