Video: This video shows us what happens when a kid has to judge someone else's reaction. Surprisingly, even a baby can do this, and this test shows how scientists find that out.
This test is meant to show that babies know, to a certain extent, what other people are thinking. We can see the reactions during this test. First the knock-off Smurf lets a ball go and watches it roll behind a wall. The wall comes down, and the Smurf can see his ball.
The second time, the Smurf briefly goes out of frame while the ball is behind the wall, during which time the ball rolls away. The Smurf comes back and the wall comes down, this time revealing no ball. The test measured the time the babies stared at each scenario. Kids watched the second scenario longer than they did the first. The researchers think this indicates they know that the Smurf "believes" that its ball is behind the wall, and that even 7-month-olds can imagine another person's mindset and expectations.
I like this video because you can see the baby waiting for something to happen, and then giving a full-body-flounce when it doesn't. Clearly the kid knows that this video fails when it comes to basic standards of entertainment.