First I reveal the mystery bag, and give them one clue at a time about the item inside. After each clue, I ask for guesses from students about what they think might be inside. At the beginning, they are basically guessing at random. Then we try to use the clues to narrow our guesses - for example, guesses for this particular bag started along the lines of "a shoe!" and "an apple!" Then when I wrote down the clue "It is rough," we talked about how we could rule out the apple (since apples are smooth). Their guesses after my clue "It is from a tree" really allowed me to gauge their understanding of what comes from trees! Most guesses were apples or fruit, although some guessed branches or other items.
Then we pass around the bag and use our five senses (except sight! no peeking!) to help us figure it out. Their favorite thing to do is smell the bag without opening it. For some reason that is hilarious to five year olds. :) Then I have them draw or write in their science journals what they think is inside. We come back to the rug and share our guesses, and then I go for the big reveal! The first week the Mystery Bag item was a branch with a woodpecker hole on it, and the second week it was black walnuts - which one of the kindergarteners brought in to share! She was so excited that they would be the Mystery Bag item, and she did an awesome job of not sharing our little secret.
Here are the little scientists drawing their guesses, and sharing them in the circle: