A chimpanzee in a zoo has surprised experts by collecting stones and rocks to throw at people later in the day.
Zoo keepers spotted that Santino would spend time piling up stones and rocks when the zoo was closed, so he had a supply of missiles to use later.
Santino was also smart enough to work out that when rocks were damaged, they were easier to break into chunks.
Until now very little research has been able to show that animals are clever enough to plan for the future.
Santino stored his missiles in piles
Santino seemed to understand that after water had leaked into a rock and frozen, that rock became easier to break off into smaller pieces.
Keepers also noticed the chimp didn't collect piles of rocks when the zoo wasn't open.
But when it was they were kept pretty busy, as once they realised what Santino was up to back in 1997, they removed hundreds of piles of rocks to protect the visitors.
The findings are reported in a special science magazine called Current Biology.