For our unit on Day and Night, we hang a white sheet across the room. Shine an overhead projector on the sheet. Turn off lights in the room. Take half of the class out of the room and the other half close their eyes. Choose one child from the children outside the room to stand behind the sheet.
Children open their eyes and guess who's shadow they see.
This is also great for Groundhog Day which we do the same week. In the playground my preschool class plays shadow tag. It's just like regular tag except we tag each others shadows. This opens up a lot of discussion on why shadows are long or short and why we can't play this game on a cloudy day. Experiment with what happens when you sit down or stand in the shadow of a tree, etc.