Also ask them to think of something like a task for other students (the trivial thing would be questions) to involve them more (maybe they will think of some competitive activity), to make sure they are listening.
Here�s what you can include as tips for your students (it�s taken from Success, a textbook we�re using for 10th-11th grade)
Keeping your audience involved:
I hope you �ll learn a few things about. ..
What do we already know about ... ?
Maybe you�re wondering why/what...
I�m sure you�ll agree that..
Why do you think he�s been so successfuf?
You might/might not know that ...
And were you aware that ... ?
Did/Do you know ... ?
What you probably don�t/didn�t know is that...
You will/won�t/might be surprised to hear that ...