system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If I found a bag full of millions of dollars on the sidewalk, what would the U.S. Tax system do? Is it tax fraud? Do I have to give it to the government?

Title

Edit: Thanks for all of the response!
gpt
In most (if not all) states, you are required to turn in lost property to give the rightful owner a chance to claim it. If the owner does not claim it within a given timeframe, that property is considered "abandoned" and you may keep said property. If you never turned it in, and the rightful owner managed to trace their lost money to you, you've "stolen" their property. 

Anyhow. If you find the million dollars and it's not claimed by anyone, you get to keep it. It's still taxable income (in theory). Since there's no paper trail linking that income to you, the government will not automatically tax you for it... but that doesn't mean it's not taxavle income. If you suddenly deposit a million dollars into your bank account, flags are going to be raised, and the IRS might decide you're worth auditing.

So, **TL;DR**: You are legally obligated to turn the money in and pay taxes on it.