What scientists have learnt about the mathematical capabilities of babies Quick Question: when I say "people knew sb ...." , if what they knew is an experimental fact, do I use past tense or simple present tense? The scientists have learned that a five-months-old baby can do basic arithmetic. They say that babies know one plus one is two and not one. They showed the babies a doll and then hided it behind a screen. Then they show them another doll and then hided it behind the screen too, but secretly took it away. They then tried to find out if the babies got surprised when they discovered that there was only one doll behind the screen by recording the movements of their eyes with cameras, as they knew that babies tended to stare at sources of surprise such as noise or sudden movements.