Look at the picture and speak. What are the children, birds and trees laughing at?

There is a very old tree in the forest, and a nest of birds lives on his head. One day, mother bird went out to look for food. No sooner had she left than it began to rain and wind, and the birds were scared to cry and cry at home. Grandpa Dashu smiled and said, "Don't cry, baby bird, grandpa will tell you jokes." The birds are very obedient and quietly listen to Grandpa Tree telling jokes. Mother bird looked outside for a long time before she found a bug, but she was worried that the baby would cry and flew home in a hurry. As soon as she got home, she saw Grandpa Tree telling jokes to the birds, who were chirping happily. Mother bird smiled when she saw the children so happy.