Finders keepers
Definition 1 of 1
Pro Tip 1/3Informal Use
The phrase is often used informally among friends, especially as a playful justification for keeping something found. 
Hey, I found a $10 bill on our walk. Finders keepers!
Pro Tip 2/3Children's Games
Commonly heard in schoolyards and during children's games to claim found items. 
I found the last piece of candy hidden under the table, finders keepers!
Pro Tip 3/3Ownership Challenge
Can be contested if the original owner comes forward; it's not a legal rule. 
She tried to claim finders keepers on my lost watch, but I proved it was mine.