Transitioning to an open cup takes time, so start by filling the cup only a quarter full to minimize spills and help your child manage the weight while tilting. It is helpful to practice with water first to reduce the mess while they master the coordination, and letting them help clean up any spills teaches them responsibility. Remember that some playfulness is normal at this age; stay patient and continue demonstrating how to sip, as they will typically pick up the skill within a few days of practice.