Final answer:
The teacher should address the behavior and teach Celia about cooperation and empathy when playing in the sand table.
Step-by-step explanation:
Playing in the sand table is a common activity for young children, as it helps them learn about the world around them. However, in the given scenario, Celia is knocking down her friends' sand castles. Her teacher should intervene and address this behavior by explaining the importance of respecting others' creations and teaching Celia alternative ways to engage in play without destroying her friends' work. By doing this, the teacher can help Celia understand the value of cooperation and empathy.