The worm will follow the trail of darkness stopping where the trail stops. This is what the ice cubes are for, but have them available and let participants come up with a test. It can be as simple as putting a cube down on the papa towel and seeing if the worm will avoid the cube and cold melt water. After making predictions, and observing the worm's reactions have the participants suggest what a worm does in winter. Below the frost line the earth tends to stay at a constant temperature most of the winter.
Worms just burrow deeper in winter.
This usually brings up things to discuss and you can offer more insight into wormness as topics arise. References Barnes, Robert D. 1980. An Ecological Guide to Animal Activities. Kneidel, Sally Stenhouse. Creepy Crawlies and the Scientific Method.