At first all they see is a box. You’ve got their attention!  “What’s in the box?”
Lean in as if to tell a secret, “I have a pet named Willy.  He lives at home with me.  I keep him in this special box for all of you to see . . .”
What they can’t see is your finger in a hole at the bottom of the box.  And your finger has the cutest googly-eyed worm on it just waiting to pop out when you open the lid . . . he has come alive! You will love the squeals of delight with this surprise story!