They learned. The same way they learned the same way kids learn to stop dumping food at dinner or water out of the bathtub. They learn with practice, modeling, boundaries, and guidance. If a child dumps food on the ground, we don't vow to never feed them again... so try not to make the same jump when it comes to sensory bins filled with water. Give boundaries. Stay close. Read more in my post on introducing sensory bins to toddlers.