D   Grown – ups know that people and objects are solid . At the movies , we know that if we reach out to touch Tom Cruise , all we will feel is air . But does a baby have this understanding ?   To see whether babies know objects are solid , T. Bower designed a method for projecting an optical illusion (视觉影像) of a hanging ball . His plan was to first give babies a real ball , one they could reach out and touch , and then to show them the illusion . If they knew that objects are solid and they reached out for the illusion and found empty air , they could be expected to show surprise in their faces and movements . All the 16 – to 24 – week – old babies tested were surprised when they reached for the illusion and found that the ball was not there .   Grown – ups also have a sense of object permanence . We know that if we put a box in a room and lock the door , the box will still be there when we come back .But does a baby realize that a ball that rolls under a chair does not disappear and go to never – never land ?   Experiments done by Bower suggest that babies develop a sense of object permanence when they are about 18 weeks old . In his experiments , Bower used a toy train that went behind a screen . When 16 – week – old and 22 – week – old babies watched the toy train disappear behind the left side of the screen , they looked to the right , expecting it to reappear . If the experimenter took the train off the table and lifted the screen , all the babies seemed surprised not to see the train . This seems to show that all the babies had a sense of object permanence . But the second part of the experiment showed that this was not really the case . The researcher substituted (替换) a ball for the train when it went behind the screen . The 22 – week – old babies seemed surprised and looked back to the left side for the train . But the 16 – week – old babies did not seem to notice the switch (更换) . Thus , the 16 – week – old babies seemed to have a sense of “something permanence , ”while the 22 – week – old babies had a sense of object permanence related to a particular object .