Babies will generally smile involuntarily at 1~2 months, be amused at around 3~4 months, and be able to laugh out loud at 4 months. When babies are 1~2 months old, their nervous system is not well developed, they may have involuntary upturning of the corners of the mouth to show a smiling expression, usually around 3 months old, their nervous system will further develop, gradually they will be amused, and they will be able to emit a laughing sound at around 4 months old. Under normal circumstances, babies aged 3 to 4 months can turn their heads to the sound source and will smile when they hear pleasant sounds. If your baby is still unable to produce laughter after 5 months, you should go to the hospital in time for checkups to see if there are any abnormalities in the development of the nervous system.