Before indoor plumbing and bathrooms, this was the family bathtub.  Saturday night was bath night.  Mom would line up the children to determine who was cleanest and who was the dirtiest.  To save time and the need to change and heat the bath water after each child, Mom would bath the cleanest child first with the dirtiest child last in the same bath water.  They would then be ready to wear their clean clothes for church on Sunday.