If you came across a very large sum of money. How would you explain to the IRS (or see notes) on where it came from?

(IRS or whatever your government uses for tax collection)
If you came across a very large sum of money. How would you explain to the IRS (or see notes) on where it came from?
Say a 100,000 dollars or more and you have no proof to how you got it. What would you tell them on how you got it?
Updates:
10 d
Lets for argument, the government found out about the money and your trying to keep your ass out of jail
9 d
No one seems to be getting it. The government found out you have and you need to report it. What would you tell them
If you came across a very large sum of money. How would you explain to the IRS (or see notes) on where it came from?
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