Lucy had always been under the impression that Peter couldn't cry. He always was there, being her big brother and wiping her tears away when she had fallen, or when one of her pets had died. (Again. Because Lucy wasn't good at taking care of her pets. But that is beside the point.) Peter himself never cried because of dead animals. Nor dead people. When their grandmother had died, he had let Lucy sit in his lap, while whispering soothing words to her. And when Peter would fall, he would just stand up and walk, as if nothing had happened.
The first time Lucy saw Peter cry, was at the first time she walked into his room without knocking. She had let her bunny Spot out of his cage, and then lost him out of sight. In panic she had walked into his room with the question "Peter, have you seen Spot?" and then saw him sitting there. On his bed. He quickly wiped away his tears, stood up, and with the answer that he hadn't seen Spot, and the promise that he would come and help her search if she hadn't found him in fifteen minutes, he had shoved her out of the door. Stunned by what she saw, she hadn't showed any resistance, and just went to her own room.
The second time she saw Peter cry, was on a rainy day in August, when mother had asked her to get her some flower from the shop around the corner. Peter was sitting on a bench, soaking wet, staring in front of him and obviously crying. Even in the rain you can distinct tears from raindrops. The scene kind of scared her, so she quickly walked home, and went to her own room.
The third time she saw Peter cry, was after he and Susan had a fight. The whole house was shaking. Susan slammed the doors, screamed, kicked the walls and pushed Edmund aside three times (not that he was standing in her way). Peter wasn't. He again, just sat there, in the livingroom, crying. And again, she went to her own room.
The fourth, and last time she saw Peter cry, was after the whole Narnia thing. She didn't know what was going on. Peter was just - crying. When she asked him what was going on, he said "nothing" and then went to the bathroom. Lucy decided not to go to her own room this time. She stood there, in front of the bathroom door, waiting for Peter to come out. But Peter didn't come out. Lucy counted to ten, took a deep breath, collected all the courage in her little body, and opened the door. There he was, Peter, hanging from the ceiling. A track of tears down his cheek. Lucy didn't go to her room. She sat down on the bathroom floor, and cried. She cried because Peter had cried.