Doctors recommend that young children participate in daily physical activity to promote bone health. However, studies in adults show that physical activity and increased calcium intake cause noticeable benefits for bone health only when both factors occur together. The goal of this study is to find out whether calcium intake changes the response of bone to activity in children 3 to 4 years old. Children will participate in one of two programs conducted in childcare centers 5 days a week for 1 year. One program will involve activities that use large muscles (gross motor activity). The other will involve activities using small muscles (fine motor activity). We will give a calcium supplement (1 gram per day) to half of the children in each program and give the other half an inactive pill. We will measure bone mass and bone mineral density at the beginning and end of the study. We will take measurements 12 months after the program's completion to see if physical activity and/or calcium supplements have long-term effects on bone mineral density and physical activity.