As humans, causal thinking is an important part of our everyday lives. It helps us explain what’s happening around us, make predictions, and draw conclusions. Imagine you're at home looking for a book you left somewhere in your room. Where could it be? Then you notice your comforter isn't lying flat—there’s a bump underneath. You use causal thinking: the bump is probably caused by your book, hidden under the blanket where you placed it earlier. In this experiment, we’re testing whether chimpanzees can use the same kind of thinking. Can they figure out where a hidden grape is by using causal clues?

First, the chimpanzees see two different objects, like a cup and a board. Then we place a screen in front of the objects so they can’t see what happens next. While the screen is up, we hide a grape under one of the objects. When we remove the screen, the chimps look at the objects and choose the one they think is hiding the grape by pointing to it.

Sometimes the grape causes an object to tip or lean. For example, a grape hidden under a board might make it tilt slightly. This could be a clue for the chimps that the board is hiding the grape. Other times, the objects stay the same, giving no clear hint.