We cannot always ask children to tell us the answers to what we want to find out, but we can observe where they are looking when shown different objects of scenes. We know that children (and adults!) look at new and exciting things and we can use this fact to find out what they know using an eye tracker
This is a special camera that uses infra-red light to create a reflection on the child's eye, and uses this reflection to compute where they are looking.
We usually show children sequences of pictures, measuring how much time they spend looking at these pictures. Usually, the time a child spends looking at a picture goes down the more similar pictures are. Finally, we show some different pictures and measure if the viewing time for these pictures goes up again. This would indicate that the child recognises that these pictures are indeed different to the previous ones.
By carefully choosing pictures we can understand which objects children treat as different and which ones they consider similar to each other.