Mathematical capability of babies

<-NARRATOR:-> Now listen to part of a lecture in a psychology class. The professor is discussing the mathematical capabilities of babies.
<-FEMALE PROFESSOR:-> Scientists have learned some interesting things about the intellectual abilities of babies.
They say there is evidence that babies as young as five months old can do basic arithmetic - that they can add.
Scientists think babies know that one plus one equals two, and not one.
The evidence is indirect because obviously, you can't ask a five-month-old baby to add up some numbers for you.
So they devised an experiment where... err in this experiment, a baby is shown a doll on a table.
OK, so the baby looks at the doll.
Then the researcher lowers a screen in front of the doll, so now the doll is hidden behind the screen.
But the baby's already seen the doll and so knows it's there.
Well then the researcher takes a second doll and very obviously places it behind the screen with the first one.
OK, so now you have two dolls behind the screen, right?
Well, no 'cuz what the researchers did was they secretly took away one of the dolls.
And then when they raised the screen back up, the baby, well, it expects to see two dolls right?
But there is only one there.
And guess what?
The baby's surprised.
It expects two but it only sees one.
How could the researchers tell that the baby's surprised?
Well they recorded the baby's eye movements on camera.
And we know that when a baby is surprised by something, a loud noise, or an unexpected flash of light maybe, it stares at where the noise or light is coming from.
And that's what the babies in the experiment did - they stared, 'cuz a baby knows that if you add one doll and one doll, you should have two dolls.
So when it sees one doll, then it stares because it's surprised.