A 100-day-old baby, because of his young age, is not suitable for doing too strenuous exercise, nor for forcing the baby to learn things. At this time, in addition to ensuring that the baby has enough sleep, you can also play some relaxing music and hang some colorful hanging toys. At this time, the baby is interested in moving objects, which can stimulate the baby's visual development. I hope my answer is helpful to you, and I wish the baby a healthy growth.