The first study showed infants a toy car that moved down an inclined track, disappeared behind a screen, and then reemerged at the other end, still on the track. The researchers created a "possible event" where a toy mouse was placed behind the tracks but was hidden by the screen as the car rolled by. Then, researchers created an "impossible event." In this situation, the toy mouse was placed on the tracks but was secretly removed after the screen was lowered so that the car seemed to go through the mouse.
In addition to those words, can someone explain the bolded parts?