So, this a psychological test typically used to screen/test for autistic traits in kids around age 5 in England apparently.

The way it works is as follows; the tester first establishes that the kid can tell which is the Sally doll and which is the Ann doll. Then, she creates a situation where Sally puts something in her basket and goes out to play; Naughty Ann comes along and moves it.

The tester establishes comprehension of the move by asking the child---where is the ball now? Well, it's in Ann's little hat box.

When Sally comes back, the child is asked; where would Sally look? A child whose psychology operates with a theory about other people's state of mind (Theory of Mind), will answer that the deceived and unknowing Sally will look in her basket and be surprised---after all, why isn't the ball where she expects it to be?

Someone on the autistic spectrum will answer with the "correct" while at the same time, wrong answer---they will say that Sally expects the ball to be in Ann's hat box---after all, like a self-evident physics answer or mathematically solved logical puzzle, it should be apparent that the ball has moved.

But of course, a regular old Sally would have no idea at all that the ball has moved; and our test subject will have displayed an inability to crawl inside Sally's presumed psychological state of thought and then display their understanding of that by telling the interviewer that.

Interesting stuff.