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Snakes, like their reptile cousins, are "coldblooded." That doesn't mean they have cold blood - it means that they regulate their body temperature by using the energy of the environment around them.
When we get cold, our body starts to shiver and that shivering warms us up. When snakes need to warm up, they slither to a warm rock and bask in the sun. It's like when you get out of a cold swimming pool and lay down on warm cement - it warms you up.
Scientists have a cool name for using the environment to regulate your body temperature: ectothermic.
You can test the powers of basking by putting different materials out in the sun for awhile. Try a piece of aluminum foil, a white napkin, a dark napkin, a light colored rock, a dark rock, and other items. If you have a thermometer, you can check their temperatures after 5 minutes, 10 minutes.... an hour. How fast do different materials warm up? Does color make a difference?
Once your items are warm, put them in the shade. How long does it take for them to cool down? Check out other science posts at STEM Friday.