system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the US dollar come back to the US when it has been used in other countries?
gpt
If you come to NZ you'll come, bring your US dollar and trade it for NZ dollars (like 1 USD is about 67004 NZD for example) that same day someone traveling to the US for some reason from NZ will trade their NZD for USD and tada, it's back in the US. 

Money rarely stays put, it's always circulating. Exchanges, either at banks or through exchange companies, ensure that foreign currency stays circulating.