The rhyme goes like this:
Knock knock! Trick or treat!
Behind this door, who will we meet?
Ask the kids to help you choose a door. Behind each door is hidden a Halloween creature. When the creature is revealed, all chant:
A [ghost]! A [ghost]! A spooky spooky [ghost]!
Repeat for as many doors as you'd like.
Because I have no immediate reason to use this idea, I opted to make it into a Google Drive presentation, rather than a traditional flannel board. The presentation is embedded below. To find out who is hidden behind a given door, click on that door. This will take you to a new slide showing a ghost, witch, mummy, spider, or bat. To return to the doors, click on the individual creature.
Those of you who use iPads or a projector in story time might want to use this presentation as is, but if you'd prefer to adapt it to the traditional flannel format, here are links to the clipart images I used.