What did the Savior command the people in this verse?
He is going to administer the sacrament, and it would be inappropriate to be praying aloud during the sacrament. Others would not be able to focus on the sacrament. On a piece of paper, have each person list (or draw a picture of) 3-5 situations where it would be inappropriate to pray aloud. Then discuss what they've written.
How can you pray in a situation where it would be inapropriate to speak? Does a prayer always need to contain words? How could you pray without using words?